[clang-tools-extra] [clang-tidy] Fix readability-identifier-naming FP on function templates (PR #198517)

[Victor Chernyakin via cfe-commits]

================
@@ -1164,30 +1165,32 @@ StyleKind IdentifierNamingCheck::findStyleKind(
     if (Decl->isAnonymousStructOrUnion())
       return SK_Invalid;
 
+    if (!Decl->isCompleteDefinition())
+      return SK_Invalid;
+
     if (const auto *Definition = Decl->getDefinition()) {
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localspook wrote:

If `isCompleteDefinition` is true, `getDefinition` won't return a null pointer, so this `if (const auto *Definition = Decl->getDefinition())` check is now redundant.

https://github.com/llvm/llvm-project/pull/198517