[IRGen] Leave _swift_dead_method_stub Internal for COFF

COFF linkers don't merge linkonce_odr symbols, leading to multiple
definition errors.

Author: DougGregor

lib/IRGen/GenDecl.cpp

@@ -2191,11 +2191,17 @@ void IRGenModule::emitVTableStubs() {
 
     if (!stub) {
       // Create a single stub function which calls swift_deletedMethodError().
+      // Use linkonce_odr hidden to merge these symbols, except on
+      // COFF where the linker cannot merge them.
+      bool canLinkOnce = !Module.getTargetTriple().isOSBinFormatCOFF();
+      auto linkage = canLinkOnce ? llvm::GlobalValue::LinkOnceODRLinkage
+                                 : llvm::GlobalValue::InternalLinkage;
       stub = llvm::Function::Create(llvm::FunctionType::get(VoidTy, false),
-                                    llvm::GlobalValue::LinkOnceODRLinkage,
-                                    "_swift_dead_method_stub",
+                                    linkage, "_swift_dead_method_stub",
                                     &Module);
-      ApplyIRLinkage(IRLinkage::InternalLinkOnceODR).to(stub);
+      ApplyIRLinkage(canLinkOnce ? IRLinkage::InternalLinkOnceODR
+                                 : IRLinkage::Internal)
+          .to(stub);
       stub->setAttributes(constructInitialAttributes());
       stub->setCallingConv(DefaultCC);
       auto *entry = llvm::BasicBlock::Create(getLLVMContext(), "entry", stub);

test/IRGen/static-vtable-stubs.swift

@@ -20,7 +20,7 @@ open class C {
 // CHECK: @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvs" = hidden alias void (), ptr @_swift_dead_method_stub
 // CHECK: @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvM" = hidden alias void (), ptr @_swift_dead_method_stub
 
-// CHECK: define linkonce_odr hidden void @_swift_dead_method_stub()
+// CHECK: define {{(linkonce_odr )?}}hidden void @_swift_dead_method_stub()
 // CHECK: entry:
 // CHECK:   tail call void @swift_deletedMethodError()
 

test/IRGen/zombies.swift

@@ -10,6 +10,6 @@ class C {
 
 // CHECK: @"$s7zombies1CC1i33_{{.*}}vs" = hidden {{(dllexport )?}}alias void (), ptr @_swift_dead_method_stub
 
-// CHECK: define linkonce_odr hidden void @_swift_dead_method_stub
+// CHECK: define {{(linkonce_odr )?}}hidden void @_swift_dead_method_stub
 // CHECK: entry:
 // CHECK:  tail call void @swift_deletedMethodError()