Sourcekit/DocSupport: fix an assertion when generating documentation for extensions with attributes

When sanitizing the documentation comments for synthesized extensions,
we expect some text like "<declaration>extension". This isn't the case
when use-facing attributes are present.

rdar://50913510

Author: nkcsgexi

lib/IDE/IDETypeChecking.cpp

@@ -39,6 +39,9 @@ static bool shouldPrintAsFavorable(const Decl *D, const PrintOptions &Options) {
   const auto *FD = dyn_cast<FuncDecl>(D);
   if (!FD)
     return true;
+  // Don't check overload choices for accessor decls.
+  if (isa<AccessorDecl>(FD))
+    return true;
   ResolvedMemberResult Result =
       resolveValueMember(*DC, BaseTy, FD->getEffectiveFullName());
   return !(Result.hasBestOverload() && Result.getBestOverload() != D);

test/SourceKit/DocSupport/Inputs/cake.swift

@@ -98,3 +98,11 @@ public struct S3<Wrapped: P5>: P5 {
     public typealias Element = Wrapped.Element
 }
 extension S3: P6 where Wrapped: P6 {}
+
+/**
+some comments
+*/
+@available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
+public extension C1 {
+  func addition() {}
+}

test/SourceKit/DocSupport/doc_swift_module.swift

@@ -1,4 +1,4 @@
 // RUN: %empty-directory(%t.mod)
-// RUN: %swift -emit-module -o %t.mod/cake.swiftmodule %S/Inputs/cake.swift -parse-as-library -enable-objc-interop
+// RUN: %swift -emit-module -o %t.mod/cake.swiftmodule %S/Inputs/cake.swift -parse-as-library  -enable-objc-interop -emit-module-doc-path %t.mod/cake.swiftdoc
 // RUN: %sourcekitd-test -req=doc-info -module cake -- -I %t.mod > %t.response
 // RUN: diff -u %s.response %t.response