devel/flang, devel/flang-clang: update to later snapshot and llvm 4.0

[jmd]

Summary

Updates flang and flang-clang to a later snapshot that uses llvm 4.0 as a base instead of llvm 3.9. Upgrades dependency accordingly.

Submitted by Rainer Hurling in PR 220829.

Test Plan

poudriere tested on amd64 10.3, 11.0, HEAD.

[swills]

Approved, looks good.