[flang][OpenMP] Upstream first part of do concurrent mapping

This PR starts the effort to upstream AMD's internal implementation of
`do concurrent` to OpenMP mapping. This replaces llvm#77285 since we
extended this WIP quite a bit on our fork over the past year.

An important part of this PR is a document that describes the current
status downstream, the upstreaming status, and next steps to make this
pass much more useful.

In addition to this document, this PR also contains the skeleton of the
pass (no useful transformations are done yet) and some testing for the
added command line options.

Author: ergawy

clang/include/clang/Driver/Options.td

@@ -6927,6 +6927,10 @@ 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-to-openmp=">,
+  HelpText<"Try to map `do concurrent` loops to OpenMP [none|host|device]">,
+      Values<"none,host,device">;
 } // let Visibility = [FC1Option, FlangOption]
 
 def J : JoinedOrSeparate<["-"], "J">,

clang/lib/Driver/ToolChains/Flang.cpp

@@ -153,7 +153,8 @@ void Flang::addCodegenOptions(const ArgList &Args,
     CmdArgs.push_back("-fversion-loops-for-stride");
 
   Args.addAllArgs(CmdArgs,
-                  {options::OPT_flang_experimental_hlfir,
+                  {options::OPT_do_concurrent_parallel_EQ,
+                   options::OPT_flang_experimental_hlfir,
                    options::OPT_flang_deprecated_no_hlfir,
                    options::OPT_fno_ppc_native_vec_elem_order,
                    options::OPT_fppc_native_vec_elem_order,