c++: Standard mangling abbreviations & modules

c++: Standard mangling abbreviations & modules

The std manglings for things like std::string should not apply if
we're not in the global module.

gcc/cp/
	* mangle.cc (is_std_substitution): Check global module.
	(is_std_substitution_char): Return bool.
gcc/testsuite/
	* g++.dg/modules/std-subst-2.C: New.
	* g++.dg/modules/std-subst-3.C: New.
	* g++.dg/modules/std-subst-4_a.C: New.
	* g++.dg/modules/std-subst-4_b.C: New.
	* g++.dg/modules/std-subst-4_c.C: New.