[clang] [flang] [mlir] [flang] Add basic -mtune support (PR #95043)

[Andrzej Warzyński via cfe-commits]

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@@ -32,6 +32,9 @@ class TargetOptions {
   /// If given, the name of the target CPU to generate code for.
   std::string cpu;
 
+  /// If given, the name of the target CPU to tune code for.
+  std::string tuneCPU;
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banach-space wrote:

I'm fine with longer self-documenting names, e.g. `cpuToTuneFor`. That would be more consistent with `cpu` above :) 

https://github.com/llvm/llvm-project/pull/95043