Add an APValue representation for the difference between two address-of-label express...

Add an APValue representation for the difference between two address-of-label expressions.  Add support to Evaluate and CGExprConstant for generating/handling them.  Remove the special-case for such differences in Expr::isConstantInitializer.

With that done, remove a bunch of buggy code from CGExprConstant for handling scalar expressions which is no longer necessary.

Fixes PR11705.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147561 91177308-0d34-0410-b5e6-96231b3b80d8