[SourceKit] Never try to report mangled names for archetypes without contexts. rdar://28094209

lib/AST/USRGeneration.cpp

@@ -31,6 +31,7 @@ static inline StringRef getUSRSpacePrefix() {
 }
 
 bool ide::printTypeUSR(Type Ty, raw_ostream &OS) {
+  assert(!Ty->hasArchetype() && "cannot have contextless archetypes mangled.");
   using namespace Mangle;
   Mangler Mangler(true);
   Mangler.mangleTypeForDebugger(Ty->getRValueType(), nullptr);

test/SourceKit/CursorInfo/crash2.swift

@@ -0,0 +1,11 @@
+protocol P {
+  func meth()
+}
+
+func foo (t : P) {
+  t.meth()
+}
+
+// RUN: %sourcekitd-test -req=cursor -pos=6:5 %s -- %s | %FileCheck %s -check-prefix=CASE1
+
+// CASE1: source.lang.swift.ref.function.method.instance (2:8-2:14)

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -658,7 +658,7 @@ static bool passCursorInfoForDecl(const ValueDecl *VD,
   unsigned MangledTypeEnd = SS.size();
 
   unsigned MangledContainerTypeStart = SS.size();
-  if (ContainerTy) {
+  if (ContainerTy && !ContainerTy->hasArchetype()) {
     llvm::raw_svector_ostream OS(SS);
     SwiftLangSupport::printTypeUSR(ContainerTy, OS);
   }