[CodeComplete] Fix mangling of protocols with primary associated types for cached type relations

#59763 changed `appendType` to no longer be able to mangle `ParameterizedProtocolType`. Instead, we need to call into `appendConstrainedExistential` directly.

This manifested as a SourceKit crash every time you invoke code completion when you import a module that exports a type alias that resolves to a protocol with a primary associated type.

rdar://98623438

Author: ahoppen

lib/AST/ASTMangler.cpp

@@ -752,6 +752,9 @@ std::string ASTMangler::mangleTypeAsUSR(Type Ty) {
 
   if (auto *fnType = Ty->getAs<AnyFunctionType>()) {
     appendFunction(fnType, nullptr);
+  } else if (Ty->is<ParameterizedProtocolType>()) {
+    // ParameterizedProtocolType cannot be mangled using 'appendType'.
+    appendConstrainedExistential(Ty, nullptr, nullptr);
   } else {
     appendType(Ty, nullptr);
   }

test/IDE/complete_from_module_with_type_alias_to_parameterized_protocol.swift

@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t/split)
+// RUN: %empty-directory(%t/build)
+// RUN: %{python} %utils/split_file.py -o %t/split %s
+
+// RUN: %target-swift-frontend -emit-module -o %t/build %t/split/pck.swift
+
+// RUN: %target-swift-ide-test -code-completion -source-filename %t/split/test.swift -I %t/build -code-completion-token=COMPLETE | %FileCheck %s
+
+// BEGIN pck.swift
+
+public protocol Foo<Bar> {
+  associatedtype Bar
+}
+
+public typealias Problem = Foo<String>
+
+// BEGIN test.swift
+
+import pck
+
+#^COMPLETE^#
+
+// CHECK: Begin completions
+// CHECK-DAG: Decl[Protocol]/OtherModule[pck]/Flair[RareType]: Foo[#Foo#];
+// CHECK-DAG: Decl[TypeAlias]/OtherModule[pck]:   Problem[#Foo<String>#];
+// CHECK: End completions