Fix regression in -I/usr/include exclusion logic
The change in commit 15ad830062 (Refactor exclusion of -I/usr/include to
avoid per-language values, 2019-01-21, v3.14.0-rc1~108^2~4) caused the
exclusion to apply to Fortran, but it was only meant for C, CXX, and
CUDA. The purpose of the change was to prepare for the value of
`CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES` to be computed from the
actual compiler instead of hard-coded. We need to preserve exclusion of
`-I/usr/include` if the compiler has any implicit include directory that
looks intended to replace it, e.g. `<sdk>/usr/include` on macOS.
Fixes: #18914
2 files changed
