c++: modules and explicit(bool) [PR103752]

c++: modules and explicit(bool) [PR103752]

We weren't streaming a C++20 dependent explicit-specifier.

	PR c++/103752

gcc/cp/ChangeLog:

	* module.cc (trees_out::core_vals): Stream explicit specifier.
	(trees_in::core_vals): Likewise.
	* pt.cc (store_explicit_specifier): No longer static.
	(tsubst_function_decl): Clear DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P.
	* cp-tree.h (lookup_explicit_specifier): Declare.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/explicit-bool-1_b.C: New test.
	* g++.dg/modules/explicit-bool-1_a.H: New test.