Reformat mir! macro invocations to use braces.

The `mir!` macro has multiple parts:
- An optional return type annotation.
- A sequence of zero or more local declarations.
- A mandatory starting anonymous basic block, which is brace-delimited.
- A sequence of zero of more additional named basic blocks.

Some `mir!` invocations use braces with a "block" style, like so:
```
mir! {
    let _unit: ();
    {
	let non_copy = S(42);
	let ptr = std::ptr::addr_of_mut!(non_copy);
	// Inside `callee`, the first argument and `*ptr` are basically
	// aliasing places!
	Call(_unit = callee(Move(*ptr), ptr), ReturnTo(after_call), UnwindContinue())
    }
    after_call = {
	Return()
    }
}
```
Some invocations use parens with a "block" style, like so:
```
mir!(
    let x: [i32; 2];
    let one: i32;
    {
	x = [42, 43];
	one = 1;
	x = [one, 2];
	RET = Move(x);
	Return()
    }
)
```
And some invocations uses parens with a "tighter" style, like so:
```
mir!({
    SetDiscriminant(*b, 0);
    Return()
})
```
This last style is generally used for cases where just the mandatory
starting basic block is present. Its braces are placed next to the
parens.

This commit changes all `mir!` invocations to use braces with a "block"
style. Why?

- Consistency is good.

- The contents of the invocation is a block of code, so it's odd to use
  parens. They are more normally used for function-like macros.

- Most importantly, the next commit will enable rustfmt for
  `tests/mir-opt/`. rustfmt is more aggressive about formatting macros
  that use parens than macros that use braces. Without this commit's
  changes, rustfmt would break a couple of `mir!` macro invocations that
  use braces within `tests/mir-opt` by inserting an extraneous comma.
  E.g.:
  ```
  mir!(type RET = (i32, bool);, { // extraneous comma after ';'
      RET.0 = 1;
      RET.1 = true;
      Return()
  })
  ```
  Switching those `mir!` invocations to use braces avoids that problem,
  resulting in this, which is nicer to read as well as being valid
  syntax:
  ```
  mir! {
      type RET = (i32, bool);
      {
	  RET.0 = 1;
	  RET.1 = true;
	  Return()
      }
  }
  ```

core/src/intrinsics/mir.rs

@@ -20,7 +20,7 @@
 //!
 //! #[custom_mir(dialect = "built")]
 //! pub fn simple(x: i32) -> i32 {
-//!     mir!(
+//!     mir! {
 //!         let temp2: i32;
 //!
 //!         {
@@ -33,7 +33,7 @@
 //!             RET = temp2;
 //!             Return()
 //!         }
-//!     )
+//!     }
 //! }
 //! ```
 //!
@@ -71,7 +71,7 @@
 //!
 //! #[custom_mir(dialect = "built")]
 //! pub fn choose_load(a: &i32, b: &i32, c: bool) -> i32 {
-//!     mir!(
+//!     mir! {
 //!         {
 //!             match c {
 //!                 true => t,
@@ -93,20 +93,22 @@
 //!             RET = *temp;
 //!             Return()
 //!         }
-//!     )
+//!     }
 //! }
 //!
 //! #[custom_mir(dialect = "built")]
 //! fn unwrap_unchecked<T>(opt: Option<T>) -> T {
-//!     mir!({
-//!         RET = Move(Field(Variant(opt, 1), 0));
-//!         Return()
-//!     })
+//!     mir! {
+//!         {
+//!             RET = Move(Field(Variant(opt, 1), 0));
+//!             Return()
+//!         }
+//!     }
 //! }
 //!
 //! #[custom_mir(dialect = "runtime", phase = "optimized")]
 //! fn push_and_pop<T>(v: &mut Vec<T>, value: T) {
-//!     mir!(
+//!     mir! {
 //!         let _unused;
 //!         let popped;
 //!
@@ -125,19 +127,19 @@
 //!         ret = {
 //!             Return()
 //!         }
-//!     )
+//!     }
 //! }
 //!
 //! #[custom_mir(dialect = "runtime", phase = "optimized")]
 //! fn annotated_return_type() -> (i32, bool) {
-//!     mir!(
+//!     mir! {
 //!         type RET = (i32, bool);
 //!         {
 //!             RET.0 = 1;
 //!             RET.1 = true;
 //!             Return()
 //!         }
-//!     )
+//!     }
 //! }
 //! ```
 //!
@@ -152,7 +154,7 @@
 //!
 //! #[custom_mir(dialect = "built")]
 //! fn borrow_error(should_init: bool) -> i32 {
-//!     mir!(
+//!     mir! {
 //!         let temp: i32;
 //!
 //!         {
@@ -171,15 +173,15 @@
 //!             RET = temp;
 //!             Return()
 //!         }
-//!     )
+//!     }
 //! }
 //! ```
 //!
 //! ```text
 //! error[E0381]: used binding is possibly-uninitialized
 //!   --> test.rs:24:13
 //!    |
-//! 8  | /     mir!(
+//! 8  | /     mir! {
 //! 9  | |         let temp: i32;
 //! 10 | |
 //! 11 | |         {
@@ -191,7 +193,7 @@
 //!    | |             ^^^^^^^^^^ value used here but it is possibly-uninitialized
 //! 25 | |             Return()
 //! 26 | |         }
-//! 27 | |     )
+//! 27 | |     }
 //!    | |_____- binding declared here but left uninitialized
 //!
 //! error: aborting due to 1 previous error
@@ -407,18 +409,22 @@ define!(
     ///
     /// #[custom_mir(dialect = "built")]
     /// fn unwrap_deref(opt: Option<&i32>) -> i32 {
-    ///     mir!({
-    ///         RET = *Field::<&i32>(Variant(opt, 1), 0);
-    ///         Return()
-    ///     })
+    ///     mir! {
+    ///         {
+    ///             RET = *Field::<&i32>(Variant(opt, 1), 0);
+    ///             Return()
+    ///         }
+    ///     }
     /// }
     ///
     /// #[custom_mir(dialect = "built")]
     /// fn set(opt: &mut Option<i32>) {
-    ///     mir!({
-    ///         place!(Field(Variant(*opt, 1), 0)) = 5;
-    ///         Return()
-    ///     })
+    ///     mir! {
+    ///         {
+    ///             place!(Field(Variant(*opt, 1), 0)) = 5;
+    ///             Return()
+    ///         }
+    ///     }
     /// }
     /// ```
     fn Field<F>(place: (), field: u32) -> F
@@ -455,7 +461,7 @@ define!(
 /// your MIR into something that is easier to parse in the compiler.
 #[rustc_macro_transparency = "transparent"]
 pub macro mir {
-    (
+    {
         $(type RET = $ret_ty:ty ;)?
         $(let $local_decl:ident $(: $local_decl_ty:ty)? ;)*
         $(debug $dbg_name:ident => $dbg_data:expr ;)*
@@ -469,7 +475,7 @@ pub macro mir {
                 $($block:tt)*
             }
         )*
-    ) => {{
+    } => {{
         // First, we declare all basic blocks.
         __internal_declare_basic_blocks!($(
             $block_name $(($block_cleanup))?