Alex Hoppen 35661f2ab6 [SourceKit] Inform consumers about cancellation from a background queue ...

This fixes a deadlock that could occur when a cursor info request is cancelled while another one is inside the cancellation callback. The schematic deadlock backtrace is

```
sourcekitd::cancelRequest
SourceKit::RequestTracker::cancel -> holds RequestTracker.RequestsMtx
anonymous func in SourceKit::SwiftASTManager::processASTAsync
SourceKit::SwiftASTConsumer::requestCancellation
anonymous func in ASTBuildOperation::addConsumer -> tries to get ASTBuildOperation.ConsumersAndResultMtx

resolveCursor
SourceKit::SwiftASTManager::processASTAsync
SourceKit::SwiftASTConsumer::requestCancellation
anonymous func in ASTBuildOperation::addConsumer
[probably inlined] ASTBuildOperation::requestConsumerCancellation -> holds ASTBuildOperation.ConsumersAndResultMtx
resolveCursor::CursorInfoConsumer::cancelled
anonymous func in SourceKit::SwiftLangSupport::getCursorInfo
anonymous func in handleRequestCursorInfo
reportCursorInfo
anonymous func in sourcekitd::handleRequest -> tries to get RequestTracker.RequestsMtx
```

rdar://110357502

2023-06-22 21:31:42 +02:00

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