[flang] Use the Flang cmake-functions to add targets.

Summary: It also removes the cycle-dependency between FortranSemantics and FortranEvaluate.

Reviewers: #flang, jdoerfert, sscalpone

Reviewed By: #flang, sscalpone

Subscribers: DavidTruby, schweitz, tskeith, mgorny, aartbik, llvm-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D78215