Refactor the expression class hierarchy for casts. Most importantly:

  - CastExpr is the root of all casts
  - ImplicitCastExpr is (still) used for all explicit casts
  - ExplicitCastExpr is now the root of all *explicit* casts
  - ExplicitCCastExpr (new name needed!?) is a C-style cast in C or C++
  - CXXFunctionalCastExpr inherits from ExplicitCastExpr
  - CXXNamedCastExpr inherits from ExplicitCastExpr and is the root of all
    of the C++ named cast expression types (static_cast, dynamic_cast, etc.)
  - Added classes CXXStaticCastExpr, CXXDynamicCastExpr, 
    CXXReinterpretCastExpr, and CXXConstCastExpr to 

Also, fixed returned-stack-addr.cpp, which broke once when we fixed
reinterpret_cast to diagnose double->int* conversions and again when
we eliminated implicit conversions to reference types. The fix is in
both testcase and SemaChecking.cpp.

Most of this patch is simply support for the renaming. There's very
little actual change in semantics.

llvm-svn: 58264