Fix determination of whether a reinterpret_cast casts away constness.

The "casts away constness" check doesn't care at all how the different
layers of the source and destination type were formed: for example, if
the source is a pointer and the destination is a pointer-to-member, the
types are still decomposed and their pointee qualifications are still
checked.

This rule is bizarre and somewhat ridiculous, so as an extension we
accept code making use of such reinterpret_casts with a warning outside
of SFINAE contexts.

llvm-svn: 336738