Split reinterpret_casts of member pointers out from CK_BitCast; this

is general goodness because representations of member pointers are
not always equivalent across member pointer types on all ABIs
(even though this isn't really standard-endorsed).

Take advantage of the new information to teach IR-generation how
to do these reinterprets in constant initializers.  Make sure this
works when intermingled with hierarchy conversions (although
this is not part of our motivating use case).  Doing this in the
constant-evaluator would probably have been better, but that would
require a *lot* of extra structure in the representation of
constant member pointers:  you'd really have to track an arbitrary
chain of hierarchy conversions and reinterpretations in order to
get this right.  Ultimately, this seems less complex.  I also
wasn't quite sure how to extend the constant evaluator to handle
foldings that we don't actually want to treat as extended
constant expressions.

llvm-svn: 150551