[v3] Add missing specializations for _Weak_result_type_impl and _Reference_wrapper_base

[Paolo Carlini]

Hi,

this resolves a relatively old, rather boring issue: we were missing
specializations matching variadic functions (at the time I pointed it
out to Doug) and cv-qualified function types (consistently with the
current std::is_function).

Tested x86_64-linux, committed to mainline.

Paolo.

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