[SourceKit] Run SwiftParser on a deep stack for the related identifiers request

We ran SwiftParser in `handlePrimaryAST` for related identifiers. That function is called on a dispatch queue with reduced stack size and could cause the parser to stack overflow if the program is very nested.

Run `handlePrimaryAST` on a deep stack for this request to fix the issue.

rdar://129960285

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -2537,6 +2537,8 @@ void SwiftLangSupport::findRelatedIdentifiersInFile(
     std::function<void(const RequestResult<RelatedIdentsResult> &)> Receiver;
     SwiftInvocationRef Invok;
 
+    bool requiresDeepStack() override { return true; }
+
 #if SWIFT_BUILD_SWIFT_SYNTAX
     // FIXME: Don't silently eat errors here.
     RelatedIdentsResult getRelatedIdents(SourceFile *SrcFile,