[DebugInfo] Emit witness and objc method declarations in debug info

When emitting a method definition in debug info, the compiler should
also emit the method's declaration, because LLVM LTO can't unify type
definitions when a child DIE is a full subprogram definition. This is
already the behavior for standard methods, this patch implements the
same behavior for witness and objc methods as well.

rdar://123334375

Author: augusto2112

lib/IRGen/IRGenDebugInfo.cpp

@@ -2834,7 +2834,9 @@ IRGenDebugInfoImpl::emitFunction(const SILDebugScope *DS, llvm::Function *Fn,
   // Because there's no good way to cross the CU boundary to insert a nested
   // DISubprogram definition in one CU into a type defined in another CU when
   // doing LTO builds.
-  if (Rep == SILFunctionTypeRepresentation::Method) {
+  if (Rep == SILFunctionTypeRepresentation::Method ||
+      Rep == SILFunctionTypeRepresentation::ObjCMethod ||
+      Rep == SILFunctionTypeRepresentation::WitnessMethod) {
     llvm::DISubprogram::DISPFlags SPFlags = llvm::DISubprogram::toSPFlags(
         /*IsLocalToUnit=*/Fn ? Fn->hasInternalLinkage() : true,
         /*IsDefinition=*/false, /*IsOptimized=*/Opts.shouldOptimize());

test/DebugInfo/lto-witness-method-declaration.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir -gdwarf-types -o - | %FileCheck %s
+
+// Verify that we added a declaration for a witness method.
+
+// CHECK: define{{.*}}@"$s4main14PutCharPrinterCAA09CharacterD0A2aDPxycfCTW"{{.*}} !dbg ![[INIT_DEF_DBG:[0-9]+]]
+// CHECK: ![[INIT_DEF_DBG]] = distinct !DISubprogram(name: "init", linkageName: "$s4main14PutCharPrinterCAA09CharacterD0A2aDPxycfCTW"
+// CHECK-SAME: DISPFlagDefinition{{.*}} declaration: ![[FUNC_DEF_DBG:[0-9]+]]
+// CHECK: ![[FUNC_DEF_DBG]] = !DISubprogram(name: "init", linkageName: "$s4main14PutCharPrinterCAA09CharacterD0A2aDPxycfCTW"
+
+protocol CharacterPrinter {
+    init()
+}
+
+class PutCharPrinter: CharacterPrinter {
+    public required init() {}
+}

test/DebugInfo/objc-witness-method-declaration.swift

@@ -0,0 +1,17 @@
+// REQUIRES: objc_interop
+// RUN: %target-swift-frontend -primary-file %s -emit-ir -gdwarf-types -o - | %FileCheck %s
+
+// Verify that we added a declaration for a witness method.
+
+// CHECK: define {{.*}}"$s4main9SomeClassC3fooyyFTo"{{.*}} !dbg ![[INIT_DEF_DBG:[0-9]+]]
+// CHECK: ![[INIT_DEF_DBG]] = distinct !DISubprogram(name: "foo", linkageName: "$s4main9SomeClassC3fooyyFTo"
+// CHECK-SAME: DISPFlagDefinition{{.*}} declaration: ![[FUNC_DEF_DBG:[0-9]+]]
+// CHECK: ![[FUNC_DEF_DBG]] = !DISubprogram(name: "foo", linkageName: "$s4main9SomeClassC3fooyyFTo"
+
+import Foundation
+
+class SomeClass {
+    public required init() {}
+
+    @objc func foo() {}
+}