[clang] [llvm] [OpenMP 60] Update parsing and semantic support for `nowait` clause to accept optional argument (PR #159628)

[Fazlay Rabbi via llvm-commits]

mdfazlay wrote:

@mjklemm - This PR is based on and supersedes #135030. I have addressed your both comments regarding the error messages (including the on about floating-point) in this PR. Please take a look and let me know if it is good now. Thank you. 

@alexey-bataev @jplehr, FYI.

https://github.com/llvm/llvm-project/pull/159628