[LTO] Fix linking with an alias defined using another alias.

[efriedma]

Summary

When we're copying an alias which will be defined later, we need to build a GlobalAlias, or else we'll crash later in IRLinker::linkGlobalValueBody.

clang sometimes constructs aliases like this for C++ destructors.

[pcc]

LGTM

llvm/trunk/lib/Linker/IRMover.cpp

if (DGV && !ShouldLink) {
     NewGV = DGV;
   } else {
     // If we are done linking global value bodies (i.e. we are performing
     // metadata linking), don't link in the global value due to this
     // reference, simply map it to null.
     if (DoneLinkingBodies)
       return nullptr;
 
-    NewGV = copyGlobalValueProto(SGV, ShouldLink);
+    NewGV = copyGlobalValueProto(SGV, ShouldLink || ForAlias);
     if (ShouldLink || !ForAlias)
       forceRenaming(NewGV, SGV->getName());
   }
 
   // Overloaded intrinsics have overloaded types names as part of their
   // names. If we renamed overloaded types we should rename the intrinsic
   // as well.
   if (Function *F = dyn_cast<Function>(NewGV))

llvm/trunk/test/LTO/Resolution/X86/Inputs/alias-alias-1.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = external global i32
+@d = global i32* @a

llvm/trunk/test/LTO/Resolution/X86/alias-alias.ll

+; RUN: llvm-as %s -o %t1.o
+; RUN: llvm-as %p/Inputs/alias-alias-1.ll -o %t2.o
+; RUN: llvm-lto2 run -o %t3.o %t1.o %t2.o -r %t2.o,a, -r %t2.o,d,px -r %t1.o,a,p -r %t1.o,c,p -r %t1.o,b -save-temps
+; RUN: llvm-dis < %t3.o.0.0.preopt.bc -o - | FileCheck %s
+; RUN: FileCheck --check-prefix=RES %s < %t3.o.resolution.txt
+
+; CHECK-NOT: alias
+; CHECK: @c = global i32 1
+; CHECK-NEXT: @d = global i32* @a
+; CHECK-EMPTY:
+; CHECK-NEXT: @a = weak alias i32, i32* @b
+; CHECK-NEXT: @b = internal alias i32, i32* @c
+
+; RES: 1.o{{$}}
+; RES-NEXT: {{^}}-r={{.*}}1.o,c,p{{$}}
+; RES-NEXT: {{^}}-r={{.*}}1.o,a,p{{$}}
+; RES-NEXT: {{^}}-r={{.*}}1.o,b,{{$}}
+; RES-NEXT: 2.o{{$}}
+; RES-NEXT: {{^}}-r={{.*}}2.o,a,{{$}}
+; RES-NEXT: {{^}}-r={{.*}}2.o,d,px{{$}}
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = weak alias i32, i32* @b
+@b = alias i32, i32* @c
+@c = global i32 1