[flang] Don't perform macro replacement unless *.F, *.F90, &c.

Avoid spurious and confusing macro replacements from things like
-DPIC on Fortran source files whose suffixes indicate that preprocessing
is not expected.

Add gfortran-like "-cpp" and "-nocpp" flags to f18 to force predefinition
of macros independent of the source file suffix.

Differential Revision: https://reviews.llvm.org/D96464