lit.cfg has never supported running .C files, so these tests were never
executed by check-clang.  Rename them to .cpp so that they run as part of
the test suite, and minorly tweak two of them that look like they were broken
when checked in to actually pass.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@227029 91177308-0d34-0410-b5e6-96231b3b80d8