Fixes https://github.com/llvm/llvm-project/issues/156232

OpenMP 6.0 12.1.2 specifies the behavior of the strict modifier for the
num_threads clause on parallel directives, along with the message and
severity clauses. This commit implements necessary codegen changes.

Reverts llvm/llvm-project#146405

OpenMP 6.0 12.1.2 specifies the behavior of the strict modifier for the
num_threads clause on parallel directives, along with the message and
severity clauses. This commit implements necessary codegen changes.

Add support for the `-fopenmp-version=60` command line argument. It is
needed for https://github.com/llvm/llvm-project/pull/119891 (`#pragma
omp stripe`) which will be the first OpenMP 6.0 directive implemented.
Add regression tests for Clang in `-fopenmp-version=60` mode.

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.)

Differential Revision:https://reviews.llvm.org/D138227

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

Differential Revision: https://reviews.llvm.org/D137209