[llvm] [Attr] Add `noipa` function attribute (PR #203304)

[J. Ryan Stinnett via llvm-commits]

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@@ -428,6 +428,7 @@ static bool hasDistinctMetadataIntrinsic(const Function &F) {
 /// Check whether \p F is eligible for function merging.
 static bool isEligibleForMerging(Function &F) {
   return !F.isDeclaration() && !F.hasAvailableExternallyLinkage() &&
+         !F.hasFnAttribute(Attribute::NoIPA) &&
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jryans wrote:

I initially tried to use `hasExactDefinition` in this pass (similar to other adjusted passes in this PR), but it appears from the `MergeFunctions` test cases that this pass is meant to activate for cases like weak linkage, which `hasExactDefinition` would prevent. So, I have conservatively only checked the new `noipa` attribute for this pass.

https://github.com/llvm/llvm-project/pull/203304