SemaCXX: Support templates in availability attributes

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262050 91177308-0d34-0410-b5e6-96231b3b80d8