Improve our handling of reference binding for subobjects of

temporaries. There are actually several interrelated fixes here:

  - When converting an object to a base class, it's only an lvalue
  cast when the original object was an lvalue and we aren't casting
  pointer-to-derived to pointer-to-base. Previously, we were
  misclassifying derived-to-base casts of class rvalues as lvalues,
  causing various oddities (including problems with reference binding
  not extending the lifetimes of some temporaries).

  - Teach the code for emitting a reference binding how to look
  through no-op casts and parentheses directly, since
  Expr::IgnoreParenNoOpCasts is just plain wrong for this. Also, make
  sure that we properly look through multiple levels of indirection
  from the temporary object, but destroy the actual temporary object;
  this fixes the reference-binding issue mentioned above.

  - Teach Objective-C message sends to bind the result as a temporary
    when needed. This is actually John's change, but it triggered the
    reference-binding problem above, so it's included here. Now John
    can actually test his return-slot improvements.

llvm-svn: 104434