[clang] Improve stack usage to increase template instantiation depth (PR #88546)

[Erich Keane via cfe-commits]

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@@ -6113,8 +6113,10 @@ static bool TryOCLZeroOpaqueTypeInitialization(Sema &S,
 InitializationSequence::InitializationSequence(
     Sema &S, const InitializedEntity &Entity, const InitializationKind &Kind,
     MultiExprArg Args, bool TopLevelOfInitList, bool TreatUnavailableAsInvalid)
-    : FailedOverloadResult(OR_Success),
-      FailedCandidateSet(Kind.getLocation(), OverloadCandidateSet::CSK_Normal) {
+    : FailedOverloadResult(OR_Success), FailedCandidateSet(nullptr) {
+  FailedCandidateSet = S.getASTContext().Allocate<OverloadCandidateSet>();
+  FailedCandidateSet = new (FailedCandidateSet) OverloadCandidateSet(
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erichkeane wrote:

As a future optimization, I wonder if we could defer this allocation until we already know that we've failed?  Perhaps in `SetOverloadFailure`?  Not sure I can grok the analysis at the moment to know how possible this is though.

https://github.com/llvm/llvm-project/pull/88546