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

[Carlos Galvez via cfe-commits]

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

Nit: typically we keep these in the header in this case, since it's just 1 more line in the header, compared to 3 more lines in the .cpp.

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