[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