[CodeGen] Restrict addEmittedDeferredDecl to incremental extensions

[Hahnfeld]

Summary

Reemission is only needed in incremental mode. With this early return,
we avoid overhead from addEmittedDeferredDecl in non-incremental mode.

[v.g.vassilev]

This LGTM but I'd like to wait for @rjmccall for a week or so.

[v.g.vassilev]

@Hahnfeld, let's move forward and rely on a post-commit review here if necessary.

[rjmccall]

Thank you, this seems reasonable.

clang/lib/CodeGen/CodeGenModule.h

void addDeferredDeclToEmit(GlobalDecl GD) {
     DeferredDeclsToEmit.emplace_back(GD);
     addEmittedDeferredDecl(GD);
   }
 
   /// Decls that were DeferredDecls and have now been emitted.
   llvm::DenseMap<llvm::StringRef, GlobalDecl> EmittedDeferredDecls;
 
   void addEmittedDeferredDecl(GlobalDecl GD) {
+    // Reemission is only needed in incremental mode.
+    if (!Context.getLangOpts().IncrementalExtensions)
+      return;
+
     // Assume a linkage by default that does not need reemission.
     auto L = llvm::GlobalValue::ExternalLinkage;
     if (llvm::isa<FunctionDecl>(GD.getDecl()))
       L = getFunctionLinkage(GD);
     else if (auto *VD = llvm::dyn_cast<VarDecl>(GD.getDecl()))
       L = getLLVMLinkageVarDefinition(VD);
 
     if (llvm::GlobalValue::isInternalLinkage(L) ||