[flang] [clang] [flang][driver] Rename `flang-new` as `flang` (PR #74377)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> It appears this PR now has conflicts that need resolving...

Thanks, I've just rebased this branch, so that should be resolved.

https://github.com/llvm/llvm-project/pull/74377