[clang] [Clang] Set `__cpp_explicit_this_parameter` (PR #107451)

[Erich Keane via cfe-commits]

erichkeane wrote:

It appears that https://github.com/llvm/llvm-project/pull/95112 is actually pretty close... are we better off holding off on this until after that goes in?  

https://github.com/llvm/llvm-project/pull/107451