Add file suffix for assembler-with-cpp.

Fixes crash when trying to recover from a crash on an assembler-with-cpp
file.  (Not sure how to write a testcase.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185562 91177308-0d34-0410-b5e6-96231b3b80d8