Implement macro expansion in IfExpr, IfExprConseqElse, IfExprConseqIf, IfExprConseqIfLet.

[antego]

Addresses #1141. Following up on #1161.

This change adds support for the macros inside the if condition expressions.

Things to note:

1. Judging by my research, the IfExprConseqIfLet isn't used. The parser treats the syntax let var = if ... as a let expression followed by if expression:
   
2. I didn't add the macro expansion to the IfLetExpr... family of expressions because I wasn't able to write a test case reproducing the missing macro expansion. I've tried the following code

fn main() -> i32 {
    let mut res = 0;

    enum E {
        X(u8),
        Y(u8),
        Z(u8),
    }
    let v = E::Y(12);
    if let E::Y(n) = v {
        res = 1;
    }

    0
}

which caused the compiler error

FAIL: rust/compile/macro43.rs (internal compiler error: in append_reference_for_def, at rust/resolve/rust-name-resolver.h:227)

[philberty]

LGTM

[CohenArthur]

Great stuff :DD Thank you for working on this!

[CohenArthur]

@antego I'd say open issues for your two notes: The first one is something that we can maybe discuss about removing (or it may be caused by a parser bug), and the second one needs to be fixed eventually. It would be really helpful :)

bors r+

[bors]

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