Sema: Check value dependent casts when possible

We know that const_cast<char *>((void)Something) is ill-formed, even if
'Something' is dependent because you can't cast from void to a pointer
type.

This fixes PR21845.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224299 91177308-0d34-0410-b5e6-96231b3b80d8