[Clang] Fix __VA_OPT__ implementation so that it treats the concatenation of a...

[Clang] Fix __VA_OPT__ implementation so that it treats the concatenation of a non-placemaker token and placemaker token as a non-placemaker token

Currently the implementation of __VA_OPT__ will treat the concatenation of a
non-placemaker token and placemaker token as a placemaker token which is not
correct. This will fix the implementation and treat the result as a
non-placemaker token.

This fixes: https://github.com/llvm/llvm-project/issues/60268

Differential Revision: https://reviews.llvm.org/D142604