[NFC][CodeGen] Use llvm::DenseMap for DeferredDecls

[yurai007]

Summary

CodeGenModule::DeferredDecls std::map operator[] seem to be hot especially while code generating huge compilation units.
In such cases using DenseMap instead gives observable compile time improvement. Patch was tested on Linux build with default config acting as benchmark.
Build was performed on isolated CPU cores in silent x86-64 Linux environment following: https://llvm.org/docs/Benchmarking.html#linux rules.
Compile time statistics diff produced by perf and time before and after change are following:
instructions -0.15%, cycles -0.7%, max-rss +0.65%.
Using StringMap instead DenseMap doesn't bring any visible gains.

[aeubanks]

https://llvm-compile-time-tracker.com/compare.php?from=c39d22d1968cf07e54b5816ba76dccef8acaace1&to=1958f5a5fb5be3fb5b6bad5079a2f3485db0b0e9&stat=instructions
mostly neutral but a tiny bit of green

seems fine to me

[erichkeane]

This looks fine to change, and seems like a slight compile timeimprovement.

It seems that the only thing we use this for is .find, .erase, and operator[], all of which don't depend on iterator order, so this should not cause any problems with determinism.

[serge-sans-paille]

The change looks fine to me. I've started a set of benchmark to make sure it provides the expected speedup.

[serge-sans-paille]

No performance regression on my side either (but only minor gains).

[yurai007]

Thank you all for review!

clang/lib/CodeGen/CodeGenModule.h

private:
   // used to remove the weak of the reference if we ever see a direct reference
   // or a definition.
   llvm::SmallPtrSet<llvm::GlobalValue*, 10> WeakRefReferences;
 
   /// This contains all the decls which have definitions but/ which are deferred
   /// for emission and therefore should only be output if they are actually
   /// used. If a decl is in this, then it is known to have not been referenced
   /// yet.
-  std::map<StringRef, GlobalDecl> DeferredDecls;
+  llvm::DenseMap<StringRef, GlobalDecl> DeferredDecls;
 
   /// This is a list of deferred decls which we have seen that *are* actually
   /// referenced. These get code generated when the module is done.
   std::vector<GlobalDecl> DeferredDeclsToEmit;
   void addDeferredDeclToEmit(GlobalDecl GD) {
     DeferredDeclsToEmit.emplace_back(GD);
   }