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()
      }
  }
  ```

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -34,14 +34,14 @@ use std::fmt;
 ///
 /// ```text
 // fn drop_term<T>(t: &mut T) {
-//     mir!(
+//     mir! {
 //         {
 //             Drop(*t, exit)
 //         }
 //         exit = {
 //             Return()
 //         }
-//     )
+//     }
 // }
 /// ```
 pub struct ElaborateDrops;