[clang-tools-extra] [clang-tidy][readability-identifier-length] refactoring and cleanup (PR #194610)

[Victor Chernyakin via cfe-commits]

================
@@ -32,19 +32,19 @@ class IdentifierLengthCheck : public ClangTidyCheck {
   const unsigned MinimumExceptionNameLength;
   const unsigned MinimumParameterNameLength;
 
-  std::string IgnoredVariableNamesInput;
+  llvm::StringRef IgnoredVariableNamesInput;
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localspook wrote:

We avoid qualifying LLVM types if possible:
```suggestion
  StringRef IgnoredVariableNamesInput;
```
(I recognize it's inconsistent with `llvm::Regex` below)

https://github.com/llvm/llvm-project/pull/194610