Add a FIXME for an unfortunate issue in ConvertBackendLocation()

This function is copying the entire file contents into memory repeatedly and
allocating new file IDs *each time* a source location is processed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211874 91177308-0d34-0410-b5e6-96231b3b80d8