c++: explicit(bool) malfunction with dependent expression [PR95066]

c++: explicit(bool) malfunction with dependent expression [PR95066]

I forgot to set DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P when merging two
function declarations and as a sad consequence, we never tsubsted
the dependent explicit-specifier in tsubst_function_decl, leading to
disregarding the explicit-specifier altogether, and wrongly accepting
this test.

	PR c++/95066
	* decl.c (duplicate_decls): Set DECL_HAS_DEPENDENT_EXPLICIT_SPEC_P.

	* g++.dg/cpp2a/explicit16.C: New test.