[flang] Use octal escapes for character literals in modfiles

Character literals can be formatted using octal or hex escapes for
non-ascii characters. This is so that the program can be unparsed for
either pgf90 or gfortran to compile. But modfiles should not be affected
by that -- they should be consistent.

This changes causes modfiles to always have character literals formatted
with octal escapes.

Differential Revision: https://reviews.llvm.org/D83703