[clang] [clang-tools-extra] [AstMatcher]`templateArgumentCountIs` support `FunctionDecl` (PR #130416)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Generally LG, but I did have a testing and commenting request.

Please also add a note to `clang/docs/ReleaseNotes.rst` so users know about the new match.

https://github.com/llvm/llvm-project/pull/130416