[flang][driver] Rename `flang-new -flang-experimental-exec` to `flang`

[everythingfunctional]

Summary

This patch renames the flang-new executable to just flang, and drops the need for the special flag -flang-experimental-exec. Thus, building flang makes the executable flang available as a usable compiler.

For reference, this was proposed nearly a year ago in https://reviews.llvm.org/D125788 and has received some new discussion and support in https://discourse.llvm.org/t/reviving-rename-flang-new-to-flang/68130.

[everythingfunctional]

Fix the flang program path

[everythingfunctional]

rebase to get fix for build/test failure

[everythingfunctional]

Address conflicts with main branch and fix new tests

[everythingfunctional]

Update with main to see if tests have been fixed.