[flang] [llvm] [flang] Implement 'F_C_STRING' library function (Fortran 2023) (PR #174474)

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

I'll run some internal testing and can help submit this PR once it's approved.

@cenewcombe , did you run llvm-test-suite with this change? There may be some gfortran tests that may need tweaking after this feature is implemented.

https://github.com/llvm/llvm-project/pull/174474