[clang-cl] why this code can be compiled with clang-cl?

[Arteiimis]

https://godbolt.org/z/EsfM9o679

minimum Example:
https://godbolt.org/z/96aeTffs3

[frederick-vs-ja]

These examples can be made accepted by Clang with -fms-extensions, but MSVC doesn't accept them.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Linpeng Zhang (Arteiimis)

https://godbolt.org/z/EsfM9o679

minimum Example:
https://godbolt.org/z/96aeTffs3