SemaCXX: Support templates in availability attributes

If the availability context is `FunctionTemplateDecl`, we should look
through it to the `FunctionDecl`.  This prevents a diagnostic in the
following case:

    class C __attribute__((unavailable));
    template <class T> void foo(C&) __attribute__((unavailable));

This adds tests for availability in templates in many other cases, but
that was the only case that failed before this patch.

I added a feature `__has_feature(attribute_availability_in_templates)`
so users can test for this.

rdar://problem/24561029

llvm-svn: 262050