Add support for Microsoft __if_exists, __if_not_exists extension at class scope.

Add support for Microsoft __if_exists, __if_not_exists extension at class scope.

Example:

typedef int TYPE;
class C {
  __if_exists(TYPE) {
     TYPE a;
  }
  __if_not_exists(TYPE) {
     this will never be parsed.
  }
};

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@132052 91177308-0d34-0410-b5e6-96231b3b80d8