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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58306 91177308-0d34-0410-b5e6-96231b3b80d8