[llvm] [flang] Enable Driver/exec.f90 test (NFC) (PR #170544)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Linking an executable requires libflang_rt.runtime.a/so, so the test would fail if it is not present. Did I forget to define "flang-rt" in lit.site.py if LLVM_ENABLE_RUNTIMES=flang-rt?

https://github.com/llvm/llvm-project/pull/170544