[NFC][flang] Inclusive language: remove instances of master

[NFC] As part of using inclusive language within the llvm project, this patch:
- replaces master with main in C++style.md to match the renaming of the master
branch,
- removes master from `FortranIR.md` where it is superfluous,
- renames a logical variable in `pre-fir-tree04.f90` containing master.

Reviewed By: ZarkoCA

Differential Revision: https://reviews.llvm.org/D113923