[llvm] Allow flang-new binary to be specified for omp fortran modules (PR #101438)

[Daniel Martinez via llvm-commits]

Calandracas606 wrote:

> LGTM, although another name might clarify what this is being used for, as FLANG_NEW_EXE might imply the name of the just built compiler executable. FLANG_NEW_HOST_EXE possibly?

Renamed the variable to `FLANG_NEW_NATIVE_EXE`

Avoided calling it "host" to avoid possible confusion with the build/host/target nomenclature.

I'll change it to `FLANG_NEW_HOST_EXE` if preferred.

https://github.com/llvm/llvm-project/pull/101438