c-family: Support #pragma region/endregion [PR85487]

These pragmas are used by some editors to mark regions of code for
grouping and folding. GCC should silently ignore them, rather than
giving -Wunknown-pragmas warnings.

	PR c/85487

gcc/ChangeLog:

	* doc/cpp/pragmas.rst (Pragmas): Document region pragmas.

gcc/c-family/ChangeLog:

	* c-pragma.cc (handle_pragma_ignore): New function.
	(init_pragma): Register region and endregion pragmas.

gcc/testsuite/ChangeLog:

	* c-c++-common/pragma-region.c: New test.