IRGen: Use llvm.compiler.used instead of llvm.used on ELF

As of "ELF: Create unique SHF_GNU_RETAIN sections for llvm.used global
objects" (https://reviews.llvm.org/D97448) LLVM will create separate
sections for symbols marked as llvm.used. Use llvm.compiler.used
instead.

rdar://82681143

lib/IRGen/GenDecl.cpp

@@ -886,6 +886,15 @@ IRGenModule::getAddrOfParentContextDescriptor(DeclContext *from,
 ///
 /// This value must have a definition by the time the module is finalized.
 void IRGenModule::addUsedGlobal(llvm::GlobalValue *global) {
+
+  // As of reviews.llvm.org/D97448 "ELF: Create unique SHF_GNU_RETAIN sections
+  // for llvm.used global objects" LLVM creates separate sections for globals in
+  // llvm.used on ELF.  Therefore we use llvm.compiler.used on ELF instead.
+  if (TargetInfo.OutputObjectFormat == llvm::Triple::ELF) {
+    addCompilerUsedGlobal(global);
+    return;
+  }
+
   LLVMUsed.push_back(global);
 }