[clang] [flang] [flang][OpenMP] Upstream first part of `do concurrent` mapping (PR #126026)

[Kareem Ergawy via cfe-commits]

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@@ -6910,6 +6910,10 @@ defm loop_versioning : BoolOptionWithoutMarshalling<"f", "version-loops-for-stri
 
 def fhermetic_module_files : Flag<["-"], "fhermetic-module-files">, Group<f_Group>,
   HelpText<"Emit hermetic module files (no nested USE association)">;
+
+def do_concurrent_parallel_EQ : Joined<["-"], "fdo-concurrent-parallel=">,
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ergawy wrote:

I somewhat agree with @mjklemm regarding the name (that's why I didn't put `openmp` at the start). However, I will leave it for the reviewers. Also, this is something that I hope will be easy to fix retrospectively if we want to; let me know if you disagree.

> PS: Does -fdo-concurrent-parallel= issue an error if actual parallelism is requested and -fopenmp is not present?

Yes, see changes in `FrontendActions.cpp`, an error message is emitted.

> PPS: If we choose to rename the thing, then is should rather be -fopenmp-parallel-do-concurrent=, since this seems to be the common naming pattern for OpenMP-related compiler flags.

In my mind, this is a `do concurrent` feature/flag not an OpenMP one. It just happens to map to OpenMP atm.

https://github.com/llvm/llvm-project/pull/126026