[clang-tools-extra] [clang-tidy][NFC] Refactor `modernize-use-trailing-return-type-check` check code and tests (PR #140759)

[Baranov Victor via cfe-commits]

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@@ -383,6 +382,10 @@ void UseTrailingReturnTypeCheck::keepSpecifiers(
   }
 }
 
+UseTrailingReturnTypeCheck::UseTrailingReturnTypeCheck(
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vbvictor wrote:

Initially, I did this change in preparation for PR https://github.com/llvm/llvm-project/pull/135383, where I added 2 options and move Ctor to .cpp file.
I'll revert this and do only obvious changes in NFC in the future to avoid confusion

https://github.com/llvm/llvm-project/pull/140759