Speculatively fix failing tests from 63816645

This was causing some Mac-specific build failures:
http://45.33.8.238/macm1/9739/step_7.txt
http://45.33.8.238/mac/31615/step_7.txt

As best I can tell with psychic debugging, the /Users/blah path to the
source file is being treated as a macro undef with the clang-cl driver.
This splits the filename off explicitly so hopefully the rest of the
command line arguments will be read properly.