AST: Allow mangling LocatableType in DWARF mode

Fixes rdar://problem/145954323.

lib/AST/ASTMangler.cpp

@@ -1379,8 +1379,12 @@ void ASTMangler::appendType(Type type, GenericSignature sig,
     case TypeKind::BuiltinUnsafeValueBuffer:
       return appendOperator("BB");
     case TypeKind::BuiltinUnboundGeneric:
-    case TypeKind::Locatable:
-      llvm_unreachable("not a real type");
+      llvm::errs() << "Don't know how to mangle a BuiltinUnboundGenericType\n";
+      abort();
+    case TypeKind::Locatable: {
+      auto loc = cast<LocatableType>(tybase);
+      return appendType(loc->getSinglyDesugaredType(), sig, forDecl);
+    }
     case TypeKind::BuiltinFixedArray: {
       auto bfa = cast<BuiltinFixedArrayType>(tybase);
       appendType(bfa->getSize(), sig, forDecl);

test/DebugInfo/Inputs/result_builder.swift

@@ -0,0 +1,13 @@
+@resultBuilder
+public struct Builder {
+  @_alwaysEmitIntoClient
+  public static func buildExpression<X>(_ x: X) -> X {x}
+  @_alwaysEmitIntoClient
+  public static func buildBlock<X>(_ x: X) -> X {x}
+}
+
+public struct View<X> {
+  public init(@Builder _: () -> X) {}
+
+  public func closure(_: () -> Void) {}
+}

test/DebugInfo/locatable_type.swift

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %S/Inputs/result_builder.swift -emit-module -emit-module-path %t/result_builder.swiftmodule
+// RUN: %target-swift-frontend -emit-ir %s -I %t -g -O
+
+// FIXME: Devise a test case that does not involve -O.
+
+import result_builder
+
+struct S {
+  static func foo(_ s: S?) -> S? {s}
+  static func foo(_ s: S) -> S {s}
+}
+
+public func test() {
+  View { View { S() } }
+  .closure {
+    let _: S? = .foo(S())
+  }
+}