c++: Adding exception specs can changed dependentness

c++:  Adding exception specs can changed dependentness

Making an exception variant can cause a non-dependent function type to
become dependent (since c++17 eh-specs are part of the type).  The
same is (possibly?) true for adding a late return type.  Fixed thusly.
My upcoming local extern-decl changes have a test case that covers
this (which was how I found it).

	gcc/cp/
	* tree.c (build_cp_fntype_variant): Clear
	TYPE_DEPENDENT_P_VALID if necessary.