[AST] Cleanup comments regarding CXXRecordDecl::isEmpty

We were missing references to the standard, some of our home-grown
verbiage didn't make any sense.

No functional change is intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270353 91177308-0d34-0410-b5e6-96231b3b80d8