c++: exception spec and stdlib specialization

We were silently accepting the pr65923.C specialization of std::swap with
the wrong exception specification; it should be declared noexcept.  Let's
limit ignoring mismatch with system headers to extern "C" functions so we
get a diagnostic for the C++ library.

In the case of an omitted exception-specification, let's also lower the
error to a pedwarn, and copy the missing spec over, to avoid a hard break
for code that accidentally relied on the old behavior.

...except extern "C" functions keep the new spec, to avoid breaking dubious
code like noexcept-type19.C.

gcc/cp/ChangeLog:

	* decl.cc (check_redeclaration_exception_specification): Remove
	OPT_Wsystem_headers from pedwarn when the old declaration is
	in a system header.  Also check std namespace.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/pr65923.C: Add noexcept.
	* g++.dg/cpp1z/aligned-new3.C: Expect pedwarn.
	* g++.dg/cpp1z/noexcept-type19.C: Add comment.

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