[flang] Remove %f18 from LIT configuration files
%f18 was originally introduced to represent the old Flang driver,
f18. With the introduction of the new driver, flang-new, we have
been switching to %flang (compiler driver) and %flang_fc1 (frontend
driver) as more generic alternatives.
As most tests have been portend to use the new LIT variables instead of
%f18, this is good time to remove it from lit.cfg.py. There's only one
test left that requires the old driver to run. It's updated with:
! REQUIRES: old-flang-driver
This way we preserve its semantics while reducing the number of
variables in LIT configuration.
Differential Revision: https://reviews.llvm.org/D101281