[clang] [flang] [mlir] [flang] Add support for -mprefer-vector-width=<value> (PR #141380)

[Eugene Epshteyn via cfe-commits]

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@@ -126,6 +127,8 @@ struct MLIRToLLVMPassPipelineConfig : public FlangEPCallBacks {
   bool NoSignedZerosFPMath =
       false; ///< Set no-signed-zeros-fp-math attribute for functions.
   bool UnsafeFPMath = false; ///< Set unsafe-fp-math attribute for functions.
+  std::string PreferVectorWidth = ""; ///< Set prefer-vector-width attribute for
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eugeneepshteyn wrote:

Don't need to initialize to empty string here, because `std::string` is by default constructed as empty string. Calling `empty()` on default-constructed string would return `true`.

https://github.com/llvm/llvm-project/pull/141380