[IRGen] Use linkonce_odr hidden linkage for _swift_dead_method_stub

IRGen introduces the symbol _swift_dead_method_stub for dead vtable
entries to point to. This symbol was given internal linkage, which
would result in multiply-defined symbols when combined with Embedded
Swift's use of aggressive CMO. Switch to linkonce_odr hidden linkage
so multiple versions of this symbol can be coalesced by the linker
(and dropped if not needed).

Fixes rdar://162392119.

Author: DougGregor

lib/IRGen/GenDecl.cpp

@@ -2192,10 +2192,11 @@ void IRGenModule::emitVTableStubs() {
     if (!stub) {
       // Create a single stub function which calls swift_deletedMethodError().
       stub = llvm::Function::Create(llvm::FunctionType::get(VoidTy, false),
-                                    llvm::GlobalValue::InternalLinkage,
-                                    "_swift_dead_method_stub");
+                                    llvm::GlobalValue::LinkOnceODRLinkage,
+                                    "_swift_dead_method_stub",
+                                    &Module);
+      ApplyIRLinkage(IRLinkage::InternalLinkOnceODR).to(stub);
       stub->setAttributes(constructInitialAttributes());
-      Module.getFunctionList().push_back(stub);
       stub->setCallingConv(DefaultCC);
       auto *entry = llvm::BasicBlock::Create(getLLVMContext(), "entry", stub);
       auto *errorFunc = getDeletedMethodErrorFn();

test/IRGen/static-vtable-stubs.swift

@@ -15,12 +15,12 @@ open class C {
   private func foo() async {}
 }
 
-// CHECK: @"$s1M1CC3foo33_{{.*}}Tu" = hidden global %swift.async_func_pointer <{ {{.*}} @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvg"
+// CHECK: @"$s1M1CC3foo33_{{.*}}Tu" = hidden global %swift.async_func_pointer <{ {{.*}} @_swift_dead_method_stub
 
-// CHECK: @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvs" = hidden alias void (), ptr @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvg"
-// CHECK: @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvM" = hidden alias void (), ptr @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvg"
+// CHECK: @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvs" = hidden alias void (), ptr @_swift_dead_method_stub
+// CHECK: @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvM" = hidden alias void (), ptr @_swift_dead_method_stub
 
-// CHECK: define hidden void @"$s1M1CC1i33_807E3D81CC6CDD898084F3279464DDF9LLSDySOypGvg"()
+// CHECK: define linkonce_odr hidden void @_swift_dead_method_stub()
 // CHECK: entry:
 // CHECK:   tail call void @swift_deletedMethodError()
 

test/IRGen/zombies.swift

@@ -8,8 +8,8 @@ class C {
   init(i: Int) { self.i = i }
 }
 
-// CHECK: @"$s7zombies1CC1i33_{{.*}}vs" = hidden {{(dllexport )?}}alias void (), ptr @"$s7zombies1CC1i33_45489CBEFBF369AB7AEE3A799A95D78DLLSivg"
+// CHECK: @"$s7zombies1CC1i33_{{.*}}vs" = hidden {{(dllexport )?}}alias void (), ptr @_swift_dead_method_stub
 
-// CHECK: define hidden void @"$s7zombies1CC1i33_45489CBEFBF369AB7AEE3A799A95D78DLLSivg"()
+// CHECK: define linkonce_odr hidden void @_swift_dead_method_stub
 // CHECK: entry:
 // CHECK:  tail call void @swift_deletedMethodError()

test/embedded/linkage/leaf_application.swift

@@ -56,13 +56,15 @@ public func unnecessary() -> Int64 { 5 }
 @_neverEmitIntoClient
 public func unusedYetThere() -> Int64 { 5 }
 
-public class PointClass {
+open class PointClass {
   public var x, y: Int
 
   public init(x: Int, y: Int) {
     self.x = x
     self.y = y
   }
+
+  private func notUsed() { }
 }
 
 public protocol Reflectable: AnyObject {
@@ -89,6 +91,9 @@ public func createsExistential() -> any Reflectable {
 
 // LIBRARY-IR-NOT: define {{.*}} @"$es27_allocateUninitializedArrayySayxG_BptBwlFSi_Tg5"
 
+
+// LIBRARY-IR: define linkonce_odr hidden void @_swift_dead_method_stub
+
 // LIBRARY-SIL: sil @$e7Library5helloSaySiGyF
 // LIBRARY-SIL: sil @$e7Library8getArraySaySiGyF : $@convention(thin) () -> @owned Array<Int> {