[SourceKit] Fix crash in getUIDForDecl() with invalid operator

We may know syntactically that we have an operator without being able to
find the associated operator decl when the input is invalid/incomplete.
Don't crash when that happens, and just fallback to a "free function"
decl kind.

rdar://problem/25196625

Author: benlangmuir

test/SourceKit/SourceDocInfo/cursor_invalid.swift

@@ -13,6 +13,15 @@ class C {
   class MyCoolClass: Undeclared {}
 }
 
+func resyncParser1() {}
+
+func ==
+func ==()
+func ==(x: C)
+func ==(x: C, y: C)
+
+func resyncParser2() {}
+
 // RUN: %sourcekitd-test -req=cursor -pos=4:13 %s -- %s | FileCheck -check-prefix=CHECK1 %s
 // CHECK1: source.lang.swift.decl.var.local (4:13-4:14)
 // CHECK1: c
@@ -38,3 +47,15 @@ class C {
 // RUN: %sourcekitd-test -req=cursor -pos=7:12 %s -- %s | FileCheck -check-prefix=EMPTY %s
 // RUN: %sourcekitd-test -req=cursor -pos=9:7 %s -- %s | FileCheck -check-prefix=EMPTY %s
 // EMPTY: <empty cursor info>
+
+// RUN: %sourcekitd-test -req=cursor -pos=18:6 %s -- %s | FileCheck -check-prefix=EQEQ1 %s
+// RUN: %sourcekitd-test -req=cursor -pos=19:6 %s -- %s | FileCheck -check-prefix=EQEQ1 %s
+// Note: we can't find the operator decl so the decl kind is a fallback.
+// EQEQ1: <decl.function.free><syntaxtype.keyword>func</syntaxtype.keyword> <decl.name>==</decl.name>()
+
+// RUN: %sourcekitd-test -req=cursor -pos=20:6 %s -- %s | FileCheck -check-prefix=EQEQ2 %s
+// Note: we can't find the operator decl so the decl kind is a fallback.
+// EQEQ2: <decl.function.free><syntaxtype.keyword>func</syntaxtype.keyword> <decl.name>==</decl.name>(<decl.var.parameter><decl.var.parameter.name>x</decl.var.parameter.name>: <decl.var.parameter.type><ref.class usr="s:C14cursor_invalid1C">C</ref.class></decl.var.parameter.type></decl.var.parameter>)</decl.function.free>
+
+// RUN: %sourcekitd-test -req=cursor -pos=21:6 %s -- %s | FileCheck -check-prefix=EQEQ3 %s
+// EQEQ3: <decl.function.operator.infix><syntaxtype.keyword>func</syntaxtype.keyword> <decl.name>==</decl.name>(<decl.var.parameter><decl.var.parameter.name>x</decl.var.parameter.name>: <decl.var.parameter.type><ref.class usr="s:C14cursor_invalid1C">C</ref.class></decl.var.parameter.type></decl.var.parameter>, <decl.var.parameter><decl.var.parameter.name>y</decl.var.parameter.name>: <decl.var.parameter.type><ref.class usr="s:C14cursor_invalid1C">C</ref.class></decl.var.parameter.type></decl.var.parameter>)</decl.function.operator.infix>

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -197,8 +197,8 @@ UIdent UIdentVisitor::visitFuncDecl(const FuncDecl *D) {
                                                IsRef);
   }
 
-  if (D->isOperator()) {
-    switch (D->getOperatorDecl()->getKind()) {
+  if (auto *Op = D->getOperatorDecl())  {
+    switch (Op->getKind()) {
     case DeclKind::PrefixOperator:
       return IsRef ? KindRefFunctionPrefixOperator : KindDeclFunctionPrefixOperator;
     case DeclKind::PostfixOperator: