Remove a broken hack from Scalar::ULongLong and fix a test

Change Test-rdar-12481949.py to expect GetValueAsUnsigned() to return
0xffffffff if the variable is an int32_t (signed, 4 byte integer) with
value of -1. The previous expectation where we expected the value to be
0xffffffffffffffff doesn't make sense as nothing explains why we would
treat it as an 8 byte value.

This CL also removes a hack from Scalar::ULongLong what was most likely
added to get this test passing as it only worked in case the value of
the variable is -1 and didn't make any sense even in that case.

Differential revision: http://reviews.llvm.org/D14611

llvm-svn: 253027