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

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.

llvm-svn: 147561