[clang] [Clang] Reapply CWG2369 "Ordering between constraints and substitution" (PR #122423)

[Erich Keane via cfe-commits]

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@@ -8054,8 +8056,9 @@ bool Sema::CheckNonDependentConversions(
   bool HasThisConversion = Method && !isa<CXXConstructorDecl>(Method);
   unsigned ThisConversions = HasThisConversion ? 1 : 0;
 
-  Conversions =
-      CandidateSet.allocateConversionSequences(ThisConversions + Args.size());
+  Conversions = Conversions.empty() ? CandidateSet.allocateConversionSequences(
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erichkeane wrote:

Rather than Ternary, I think this should just be;

`if (Conversions.empty()) Conversions = CandidateSet....`

https://github.com/llvm/llvm-project/pull/122423