[clang] [z/OS] Add option to target older versions of LE on z/OS (PR #123399)

[Abhina Sree via cfe-commits]

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@@ -277,6 +277,11 @@ def err_cpu_unsupported_isa
 def err_arch_unsupported_isa
   : Error<"architecture '%0' does not support '%1' execution mode">;
 
+def err_zos_target_release_discontinued
+  : Error<"z/OS target level \"%0\" is discontinued.  Unexpected behavior might occur if an out-of-support target level is specified.  Use z/OS target level \"zOSv2r4\", or later instead">;
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abhina-sree wrote:

minor nit: these lines are very long and could be broken up to multiple lines

https://github.com/llvm/llvm-project/pull/123399