Exclude calls to variadic lambda stubs from -Wnonnull checking (PR c++/95984).

Exclude calls to variadic lambda stubs from -Wnonnull checking (PR c++/95984).

Resolves:
PR c++/95984 - Internal compiler error: Error reporting routines re-entered in -Wnonnull on a variadic lamnda
PR c++/96021 - missing -Wnonnull passing nullptr to a nonnull variadic lambda

gcc/c-family/ChangeLog:

	PR c++/95984
	* c-common.c (check_function_nonnull): Avoid checking syntesized calls
	to stub lambda objects with null this pointer.
	(check_nonnull_arg): Handle C++ nullptr.

gcc/cp/ChangeLog:

	PR c++/95984
	* call.c (build_over_call): Check calls only when tf_warning is set.

gcc/testsuite/ChangeLog:

	PR c++/95984
	* g++.dg/warn/Wnonnull6.C: New test.