Re-enabled support for the Subjects for the weak attribute. This changes the diagnostic involved to be more accurate -- for C++ code, it will now report that weak applies to variables, functions or classes. Added additional test case for this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196120 91177308-0d34-0410-b5e6-96231b3b80d8