[clang] [clang][Sema] Don't warn for implicit uses of builtins in system headers (PR #138205)

[Nick Sarnie via cfe-commits]

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@@ -1825,6 +1825,11 @@ class Sema final : public SemaBase {
   /// Set of no-builtin functions listed by \#pragma function.
   llvm::SmallSetVector<StringRef, 4> MSFunctionNoBuiltins;
 
+  /// Map of BuiltinIDs to source locations that have #pragma intrinsic calls
+  /// that refer to them.
+  llvm::DenseMap<unsigned, llvm::SmallSetVector<SourceLocation, 4>>
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sarnex wrote:

I tried compiling the system header into a PCH and [dumping](https://gist.github.com/sarnex/2ae0c9c543a004622734a6cb1bdead0a) the AST with `llvm-bcanalyzer` and I didn't see any implicit declarations, not sure if I was checking for the right thing.



Either way, I tried another solution where we declare the builtin if it wasn't already delcared. Fixes the ambiguous call errors I saw when trying to universally declare it and fixes the original issue. Please take a look when you have a sec, thanks.

https://github.com/llvm/llvm-project/pull/138205