lowering: split finally blocks for exceptional control-flow

[topolarity]

This change duplicates finally blocks in lowered IR, so that they can have a static nesting depth in the try-catch hierarchy.

Previously, finally control-flow looked like this:

error   non-error
    \   /
     \ /
      |
    finally block
      |
     / \
    /   \
 error   non-error

This kind of flow is a problem, because in a couple places the compiler assumes that it can actually "color" the CFG such that there is a static nesting depth at each BasicBlock (i.e. each BasicBlock can be labeled w/ a unique enclosing try / catch scope). The above finally pattern violates that assumption.

In an upcoming PR, I want to extend the lifetimes of our Event Handlers (jl_handler_t) until the end of a catch block (rather than the start) which noticeably breaks llvm-lower-handlers.cpp. (@Keno was very clear about this assumption in the comments for that pass.)

Behaviorally this was mostly benign, except for some mis-handling of an erroring entry that turns into a non-erroring exit:

function foo()
    try
        error("foo")
    finally
        return nothing
    end
end

function bar()
    if foo() === nothing
        # ...
    end
    # do things
    error("baz")
end

which spits out internal compiler stack traces (oops!):

julia> bar()
ERROR: baz
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] bar()
   @ Main ./REPL[2]:6
 [3] top-level scope
   @ REPL[3]:1

caused by: foo
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:35
  [2] foo()
    @ Main ./REPL[1]:3
  [3]
    @ Core.Compiler ./compiler/abstractinterpretation.jl:909
  [4]
    @ Core.Compiler ./compiler/abstractinterpretation.jl:794
  [5]
    @ Core.Compiler ./compiler/abstractinterpretation.jl:788
  [6]
    @ Core.Compiler ./compiler/abstractinterpretation.jl:103
# ... <snip>
 [21] typeinf_ext_toplevel(mi::Core.MethodInstance, world::UInt64)
    @ Core.Compiler ./compiler/typeinfer.jl:1078
 [22] top-level scope
    @ REPL[3]:1

That could be fixed by banning break and return within finally blocks or making the lowering more complicated, but this PR instead splits the finally block into an erroring and non-erroring path so that we can attach the catch handler appropriately.

[topolarity]

Planning on landing this today or tomorrow, unless there are any objections.

[nsajko]

This causes spurious syntax errors: #56904