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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332368 91177308-0d34-0410-b5e6-96231b3b80d8
