[libcxx] [test] Rename __c to ch. NFCI.

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@310759 91177308-0d34-0410-b5e6-96231b3b80d8
37 files changed