Sema: Methods in unavailable classes are unavailable

Similar to the template cases in r262050, when a C++ method in an
unavailable struct/class calls unavailable API, don't diagnose an error.
I.e., this case was failing:

    void foo() __attribute__((unavailable));
    struct __attribute__((unavailable)) A {
      void bar() { foo(); }
    };

Since A is unavailable, A::bar is allowed to call foo.  However, we were
emitting a diagnostic here.  This commit checks up the context chain
from A::bar, in a manner inspired by SemaDeclAttr.cpp:isDeclUnavailable.

I expected to find other related issues but failed to trigger them:

- I wondered if DeclBase::getAvailability should check for
  `TemplateDecl` instead of `FunctionTemplateDecl`, but I couldn't find
  a way to trigger this.  I left behind a few extra tests to make sure
  we don't regress.

- I wondered if Sema::isFunctionConsideredUnavailable should be
  symmetric, checking up the context chain of the callee (this commit
  only checks up the context chain of the caller).  However, I couldn't
  think of a testcase that didn't require first referencing the
  unavailable type; this, we already diagnose.

rdar://problem/25030656

llvm-svn: 262921