[SourceKit] Don't forget to check for nullptr

After 7400d48 we tried to walk into enum elements, but forgot to check
for missing types (which caused an assertion in getType) or element
decls (which caused an assertion or crash inside passReference).

rdar://problem/24634223

Author: benlangmuir

lib/IDE/SourceEntityWalker.cpp

@@ -311,7 +311,11 @@ bool SemaAnnotator::walkToTypeReprPost(TypeRepr *T) {
 
 std::pair<bool, Pattern *> SemaAnnotator::walkToPatternPre(Pattern *P) {
   if (auto *EP = dyn_cast<EnumElementPattern>(P)) {
-    return { passReference(EP->getElementDecl(), EP->getType(), DeclNameLoc(EP->getLoc())), P };
+    auto *Element = EP->getElementDecl();
+    if (!Element)
+      return { true, P };
+    Type T = EP->hasType() ? EP->getType() : Type();
+    return { passReference(Element, T, DeclNameLoc(EP->getLoc())), P };
   }
 
   auto *TP = dyn_cast<TypedPattern>(P);

test/SourceKit/Indexing/index_enum_case.swift

@@ -19,3 +19,14 @@ public enum E {
 }
 
 let e: E = .two(a:"String")
+
+func brokenEnums() {
+  switch NonExistent.A {
+  case .A:
+    return "one"
+  }
+  switch E.one {
+  case .tenthousand:
+    return "one"
+  }
+}

test/SourceKit/Indexing/index_enum_case.swift.response

@@ -128,6 +128,29 @@
       key.usr: "s:FO15index_enum_case1E3twoFMS0_FT1aSS_S0_",
       key.line: 21,
       key.column: 13
+    },
+    {
+      key.kind: source.lang.swift.decl.function.free,
+      key.name: "brokenEnums()",
+      key.usr: "s:F15index_enum_case11brokenEnumsFT_T_",
+      key.line: 23,
+      key.column: 6,
+      key.entities: [
+        {
+          key.kind: source.lang.swift.ref.enum,
+          key.name: "E",
+          key.usr: "s:O15index_enum_case1E",
+          key.line: 28,
+          key.column: 10
+        },
+        {
+          key.kind: source.lang.swift.ref.enumelement,
+          key.name: "one",
+          key.usr: "s:FO15index_enum_case1E3oneFMS0_S0_",
+          key.line: 28,
+          key.column: 12
+        }
+      ]
     }
   ]
 }