Rename three cxx files in unittests to cpp.

LLVM uses cpp as its C++ file extension, these are the only three cxx file in
the monorepo. These files apparently were called to escape a CMake check -- use
the LLVM_OPTIONAL_SOURCES mechanism that's meant as an escape for this case
instead.

No intended behavior change.
https://reviews.llvm.org/D46843

llvm-svn: 332368