devel/flang and devel/flang-clang: Update to snapshots supporting llvm 4.0 and upgrade dependencies to this version.

PR: 220829
Submitted by: Rainer Hurling (rhurlin@gwdg.de)
Reviewed by: swills (mentor)
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D11709