[flang] Switch from %f18 to %flang_fc1 in a test

This patch updates the final test that can be shared between the old and
the new Flang drivers and that has not been ported yet. %f18 (always
expanded as `f18`) is replaced with %flang_fc1 (expanded as either `f18`
or `flang-new -fc1`, depending on `FLANG_BUILD_NEW_DRIVER`).

This test should've been updated in https://reviews.llvm.org/D100309,
but I missed it then. That's because this test contains non-ascii
characters and `grep -I %f18` (as well as other grep-like tools) skips
it because it's interpreted as a data/binary file. In fact, it's just a
text file with non-ascii chars.

Since this is an obvious omission from D100309 (reviewed, accepted and
merged), I'm sending this without a review to reduce the noise on
Phabricator.