[Bug fortran/79165] 100% compile-time increase for polyhedron aermod by r244581

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79165

--- Comment #18 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
I wonder if it's worth injecting the equivalent of -fno-diagnostics-show-caret
into the Fortran FE for when it's throwing away diagnostics, perhaps by turning
gfc_buffer_error's bool flag into a tri-state:
  * unbuffered
  * buffered
  * discarderd

With the bogus linemap removed, median of 3 runs: 

real    0m38.898s
user    0m38.234s
sys     0m0.505s

and median of 3 runs with -fno-diagnostics-show-caret:

real    0m38.400s
user    0m37.729s
sys     0m0.539s

so adding -fno-diagnostics-show-caret makes it take only 98.7% of the wall time
of the old.