[libcxx] [test] Rename _Up to U, etc. NFCI.

This improves readability and (theoretically) improves portability,
as _Ugly names are reserved.

This performs additional de-uglification, so all of these tests
follow the example of iterator.traits/empty.pass.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@310761 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed