Added code to support use of "this" and "self" in

expressions.  This involved three main changes:

 - In ClangUserExpression::ClangUserExpression(),
   we now insert the following lines into the
   expression:
     #define this ___clang_this
     #define self ___clang_self

 - In ClangExpressionDeclMap::GetDecls(), we
   special-case ___clang_(this|self) and instead
   look up "this" or "self"

 - In ClangASTSource, we introduce the capability
   to generate Decls with a different, overridden,
   name from the one that was requested, e.g.
   this for ___clang_this.

llvm-svn: 113866