[clang] [flang] [llvm] [openmp] [flang][driver] rename flang-new to flang (PR #110023)

[Sylvestre Ledru via cfe-commits]

sylvestre wrote:

@everythingfunctional with this change and install, we are installing flang with the -20 in the install dir:
` -- Installing: /build/source/debian/tmp/usr/lib/llvm-20/bin/flang-20 `

it should not be the case, it should be named `/usr/lib/llvm-20/bin/flang` to match what is done elsewhere

https://github.com/llvm/llvm-project/pull/110023