[clang] [Clang] Instantiate constexpr function when they are needed. (PR #173537)

[Corentin Jabot via cfe-commits]

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@@ -786,6 +797,8 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// Keeps track of the deallocated DeclListNodes for future reuse.
   DeclListNode *ListNodeFreeList = nullptr;
 
+  std::unique_ptr<SemaProxy> SemaProxyPtr;
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cor3ntin wrote:

See the description of the PR.
For reflection, we will need to pass around (or put on the stack or something) some sort of additional context (and/or modify EvaluationContext, which would be better).

It's important to separate "do we have access to sema" from "can we do that specific thing" - notably because sema can do different things some but not all of which can be valid in a particular context - here we need to check for example that we are in a constant context, not doing trial evaluation, not checking for overflow, etc  

https://github.com/llvm/llvm-project/pull/173537