[clang] [-Wunsafe-buffer-usage] Fix a bug and suppress libc warnings for C files (PR #109496)

[Ziqing Luo via cfe-commits]

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@@ -784,12 +786,12 @@ AST_MATCHER_P(CallExpr, hasUnsafePrintfStringArg,
     return false; // possibly some user-defined printf function
 
   ASTContext &Ctx = Finder->getASTContext();
-  QualType FristParmTy = FD->getParamDecl(0)->getType();
+  QualType FirstParmTy = FD->getParamDecl(0)->getType();
 
-  if (!FristParmTy->isPointerType())
+  if (!FirstParmTy->isPointerType())
     return false; // possibly some user-defined printf function
 
-  QualType FirstPteTy = (cast<PointerType>(FristParmTy))->getPointeeType();
+  QualType FirstPteTy = FirstParmTy->getAs<PointerType>()->getPointeeType();
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ziqingluo-90 wrote:

The difference between `castAs` and `getAs` is that `castAs` asserts the canonical type is `PointerType` while `getAs` returns null if not.  I think `castAs` is more suitable for our case.

https://github.com/llvm/llvm-project/pull/109496