[Flang] add flang as a new subproject in cmake

Summary: This patch is some minor prep work for merging the flang(f18) project into the monorepo.  This patch adds "flang" as a supported target for the LLVM_ENABLE_PROJECTS option.

Reviewers: fhahn, tstellar, jdoerfert, beanz, DavidTruby

Reviewed By: DavidTruby

Subscribers: hfinkel, DavidTruby, aartbik, mgorny, llvm-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D72416