Improve our handling of (C++) references within Clang. Specifically:

  - Do not allow expressions to ever have reference type
  - Extend Expr::isLvalue to handle more cases where having written a
    reference into the source implies that the expression is an lvalue
    (e.g., function calls, C++ casts).
  - Make GRExprEngine::VisitCall treat the call arguments as lvalues when
    they are being bound to a reference parameter.

llvm-svn: 58306