[SourceKit] Use a deep stack to perform syntactic parsing

Othwerise we were performing the syntactic parsing on a background queue that had a reduced stack size which could result in stack overflows.

rdar://84474387

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -735,8 +735,17 @@ public:
 
   void parseIfNeeded() {
     if (!IsParsed) {
-      Parser->parse();
-      IsParsed = true;
+      // Perform parsing on a deep stack that's the same size as the main thread
+      // during normal compilation to avoid stack overflows.
+      static WorkQueue BigStackQueue{
+          WorkQueue::Dequeuing::Concurrent,
+          "SwiftDocumentSyntaxInfo::parseIfNeeded.BigStackQueue"};
+      BigStackQueue.dispatchSync(
+          [this]() {
+            Parser->parse();
+            IsParsed = true;
+          },
+          /*isStackDeep=*/true);
     }
   }