c++: Disable -Wignored-qualifiers for template args [PR107492]

c++: Disable -Wignored-qualifiers for template args [PR107492]

It seems wrong to issue a -Wignored-qualifiers warning for code like:

  static_assert(!is_same_v<void(*)(), const void(*)()>);

because there the qualifier matters.  Likewise in template
specialization:

  template<typename T> struct S { };
  template<> struct S<void(*)()> { };
  template<> struct S<const void(*)()> { }; // OK, not a redefinition

And likewise in other type-id contexts such as trailing-return-type:

  auto g() -> const void (*)();

This patch limits the warning to the function declaration context only.

	PR c++/107492

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Only emit a -Wignored-qualifiers warning
	when funcdecl_p.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wignored-qualifiers3.C: New test.