[sourcekit] Use a shared_ptr for the SwiftASTManager

When the server shuts down we may still have outstanding async work to
build an AST, so use a shared_ptr + weak_ptr instead of unique_ptr +
unowned references.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.cpp

@@ -178,7 +178,7 @@ SwiftLangSupport::SwiftLangSupport(SourceKit::Context &SKCtx)
   llvm::sys::path::append(LibPath, "swift");
   RuntimeResourcePath = LibPath.str();
 
-  ASTMgr.reset(new SwiftASTManager(*this));
+  ASTMgr = std::make_shared<SwiftASTManager>(*this);
   // By default, just use the in-memory cache.
   CCCache->inMemory = llvm::make_unique<ide::CodeCompletionCache>();
 }