In Clang 16, we implemented the ability to add a label at the end of a
compound statement. These changes complete the implementation by
allowing a label to be followed by a declaration in C.
Note, this seems to have fixed an issue with some OpenMP stand-alone
directives not being properly diagnosed as per:
https://www.openmp.org/spec-html/5.1/openmpsu19.html#x34-330002.1.3
(The same requirement exists in OpenMP 5.2 as well.)
Error directive is allowed in both declared and executable contexts.
The function ActOnOpenMPAtClause is called in both places during the
parsers.
Adding a param "bool InExContext" to identify context which is used to
emit error massage.
Differential Revision: https://reviews.llvm.org/D137851
