[flang] Flush and master constructs

This patch adds tests for flush and master constructs

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D122250

Co-authored By: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>