Harden/relax clang/test/CodeGen/opt-record-MIR.c test

Summary:
If the build path is short, `Line` field can end up fitting on the same line as `File`,
but the `{{.*}}` would consume it. Keeping in mind rL293149, i think we can fix it,
while keeping it working when there are and there are not any quotations.
At least this fixes this test for me.

Reviewers: anemet, aaron.ballman, hfinkel

Reviewed By: anemet

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D49348

llvm-svn: 337249