Submitted by: Bill Wendling

Reviewed by: Chris Lattner

- Fix for C++ casting operators failing during parsing. Deriving the C++ cast
  expressions from CastExpr was the wrong way to go. Its constructor creates
  null QualTypes in one of its constructors. This doesn't work well with how
  the C++ casting expression class wanted to do things. Derive just from Expr
  instead.

llvm-svn: 39714