[LTO][test] Improve comdat-mixed-lto.ll after D135427

[MaskRay]

Since D135427 was reverted, I just folded the test into the to-be-relanded D135427

llvm/lib/Transforms/IPO/FunctionImport.cpp

void llvm::thinLTOFinalizeInModule(Module &TheModule,
   // For a non-prevailing comdat, all its members must be available_externally.
   // FinalizeInModule has handled non-local-linkage GlobalValues. Here we handle
   // local linkage GlobalValues.
   if (NonPrevailingComdats.empty())
     return;
   for (auto &GO : TheModule.global_objects()) {
     if (auto *C = GO.getComdat(); C && NonPrevailingComdats.count(C)) {
       GO.setComdat(nullptr);
-      assert(GO.hasLocalLinkage() && "GO's comdat should have been dropped");
       GO.setLinkage(GlobalValue::AvailableExternallyLinkage);
     }
   }
 }
 
 /// Run internalization on \p TheModule based on symmary analysis.
 void llvm::thinLTOInternalizeModule(Module &TheModule,
                                     const GVSummaryMapTy &DefinedGlobals) {

llvm/test/LTO/Resolution/X86/comdat-mixed-lto.ll

 ; RUN: llvm-dis %t3.0.0.preopt.bc -o - | FileCheck %s
 
 ; CHECK: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @__cxx_global_var_init, ptr @C }]
 ; CHECK: @C = available_externally dso_local global %"class.Test::ptr" zeroinitializer, align 4
 ; CHECK-NOT: declare
 ; CHECK: declare dso_local void @__cxx_global_var_init() section ".text.startup"
 ; CHECK-NOT: declare
 
+; Check the behavior with the prevailing testglobfunc in %t2.o.
+; RUN: llvm-lto2 run -r=%t1.o,C,pl -r=%t2.o,C,l -r=%t1.o,testglobfunc,lx -r=%t2.o,testglobfunc,plx -o %t4 %t1.o %t2.o -save-temps
+; RUN: llvm-dis %t4.0.0.preopt.bc -o - | FileCheck %s --check-prefix=CHECK2
+
+; CHECK2: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @__cxx_global_var_init, ptr @C }]
+; CHECK2: @C = available_externally dso_local global %"class.Test::ptr" zeroinitializer, align 4
+; CHECK2: declare dso_local void @__cxx_global_var_init() section ".text.startup"
+; CHECK2: define available_externally dso_local void @testglobfunc() section ".text.startup" {
+
 ; ModuleID = 'comdat-mixed-lto.o'
 source_filename = "comdat-mixed-lto.cpp"
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 %"class.Test::ptr" = type { i32 }
 
 $C = comdat any