[clang] [clang-cl] Fix value of __FUNCTION__ in MSVC mode. (PR #67592)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

The failure found by CI looks to be related:
```
<stdin>:422:1: note: possible intended match here
VarDecl:{ResultType struct struct X}{TypedText f1} (50) (deprecated)
```
(note the `struct struct`).

https://github.com/llvm/llvm-project/pull/67592