c++: Adding exception specs can changed dependentness

[Nathan Sidwell]

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.

nathan

-- 
Nathan Sidwell
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