[Mangling] Add a new mangling for opaque return type to use when mangling an ObjC runtime name

docs/ABI/Mangling.rst

@@ -654,6 +654,11 @@ implementation details of a function type.
     opaque-type-decl-name ::= entity 'QO' // opaque result type of specified decl
   #endif
 
+  #if SWIFT_VERSION >= 5.4
+    type ::= 'Qu'                         // opaque result type (of current decl)
+                                          // used for ObjC class runtime name purposes.
+  #endif
+
 Opaque return types have a special short representation in the mangling of
 their defining entity. In structural position, opaque types are fully qualified
 by mangling the defining entity for the opaque declaration and the substitutions

lib/Demangling/OldDemangler.cpp

@@ -2010,6 +2010,10 @@ class OldDemangler {
       }
     }
     if (c == 'Q') {
+      if (Mangled.nextIf('u')) {
+        // Special mangling for opaque return type.
+        return Factory.createNode(Node::Kind::OpaqueReturnType);
+      }
       return demangleArchetypeType();
     }
     if (c == 'q') {

lib/Demangling/OldRemangler.cpp

@@ -2106,7 +2106,7 @@ void Remangler::mangleSugaredParen(Node *node) {
 }
 
 void Remangler::mangleOpaqueReturnType(Node *node) {
-  unreachable("unsupported");
+  Buffer << "Qu";
 }
 void Remangler::mangleOpaqueReturnTypeOf(Node *node, EntityContext &ctx) {
   unreachable("unsupported");

test/Demangle/Inputs/manglings.txt

@@ -329,6 +329,7 @@ _$S3BBBBf0602365061_ ---> _$S3BBBBf0602365061_
 _$S3BBBBi0602365061_ ---> _$S3BBBBi0602365061_
 _$S3BBBBv0602365061_ ---> _$S3BBBBv0602365061_
 _T0lxxxmmmTk ---> _T0lxxxmmmTk
+_TtCF4test11doNotCrash1FT_QuL_8MyClass1 ---> MyClass1 #1 in test.doNotCrash1() -> some
 $s4Test5ProtoP8IteratorV10collectionAEy_qd__Gqd___tcfc ---> Test.Proto.Iterator.init(collection: A1) -> Test.Proto.Iterator<A1>
 $s4test3fooV4blahyAA1SV1fQryFQOy_Qo_AHF ---> test.foo.blah(<<opaque return type of test.S.f() -> some>>.0) -> <<opaque return type of test.S.f() -> some>>.0
 $S3nix8MystructV1xACyxGx_tcfc7MyaliasL_ayx__GD ---> Myalias #1 in nix.Mystruct<A>.init(x: A) -> nix.Mystruct<A>

validation-test/compiler_crashers_2_fixed/sr13203.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -disable-availability-checking -emit-ir -o /dev/null %s
+
+protocol MyProtocol {}
+
+func doNotCrash1() -> some MyProtocol {
+  class MyClass1: MyProtocol {}
+  return MyClass1()
+}
+
+var doNotCrash2: some MyProtocol {
+  class MyClass2: MyProtocol {}
+  return MyClass2()
+}