[SourceKit] Add a request to get the semantic tokens of a file

We need this request for semantic highlighting in LSP. Previously, we were getting the semantic tokens using a 0,0 edit after a document update notification but that will no longer be possible if we open the documents in syntactic only mode.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h

@@ -665,6 +665,13 @@ public:
                  std::function<void(const RequestResult<DiagnosticsResult> &)>
                      Receiver) override;
 
+  void getSemanticTokens(
+      StringRef PrimaryFilePath, StringRef InputBufferName,
+      ArrayRef<const char *> Args, llvm::Optional<VFSOptions> VfsOptions,
+      SourceKitCancellationToken CancellationToken,
+      std::function<void(const RequestResult<SemanticTokensResult> &)> Receiver)
+      override;
+
   void getNameInfo(
       StringRef PrimaryFilePath, StringRef InputBufferName, unsigned Offset,
       NameTranslatingInfo &Input, ArrayRef<const char *> Args,
@@ -812,6 +819,10 @@ public:
 /// Disable expensive SIL options which do not affect indexing or diagnostics.
 void disableExpensiveSILOptions(swift::SILOptions &Opts);
 
+swift::SourceFile *retrieveInputFile(StringRef inputBufferName,
+                                     const swift::CompilerInstance &CI,
+                                     bool haveRealPath = false);
+
 } // namespace SourceKit
 
 #endif