flang: Fix module-debug-file-loc-linux.f90 in standalone

In a standalone build, there is no guarantee that flang code would be
saved in a directory named flang. Check only the path under flang's root
directory.

Reviewed By: #flang, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D157642

(cherry picked from commit 94f377d8)