devel/flang[-clang]: update to later snapshots

This introduces flang-driver, a clang6-derived driver / companion compiler, and
uses math/libpgmath as the math library backend. Notably, the latter currently
limits us to 12-CURRENT.

Patches are a combination of carry-over and adapted OpenBSD ones.

While there, pet portlint.

Reviewed by: zeising (mentor), mat
Approved by: zeising (mentor)
Obtained from: Brian Callahan (bcallah@openbsd.org) with adaptations
Differential Revision: https://reviews.freebsd.org/D17036