Scalar::MakeUnsigned was implemented incorrectly so it didn't
really change the sign of the type (leaving signed types signed).
This showed up as a misevaluation when IR-interpreting urem but
it's likely to arise in other contexts.
This commit fixes the definition, and adds a test to make
sure this won't regress in future (hopefully).
Fixes rdar://problem/42038760 and LLVM PR38076
Differential Revision: https://reviews.llvm.org/D49155
llvm-svn: 336872
