Rename test.cc files to test.cpp.

The lit.cfg files only add .cpp to suffixes, so these tests used to never run,
oops.  (Also tweak to of these tests in minor ways to make the actually pass.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224718 91177308-0d34-0410-b5e6-96231b3b80d8