Fix implementation of C++'s restrictions on using-declarations referring to enumerators:

Fix implementation of C++'s restrictions on using-declarations referring to enumerators:

 * an unscoped enumerator whose enumeration is a class member is itself a class
   member, so can only be the subject of a class-scope using-declaration.

 * a scoped enumerator cannot be the subject of a class-scope using-declaration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268594 91177308-0d34-0410-b5e6-96231b3b80d8