Cache calculated file state in LSP (#4897)

Add caching of parsed documents, and testing of the textDocument
handlers. This is based on #4896, which splits out some of the
boilerplate to calls.

Note, this caches the entire parse state because we'll want to try to
emit diagnostics when we see the update, without waiting. It may be
helpful to do that asynchronously, but we don't want to wait for another
call (such as documentSymbol). Really, we'll probably want to also add
check for diagnostics, at least.

toolchain/source/source_buffer.h

@@ -57,6 +57,12 @@ class SourceBuffer {
     }
   }
 
+  // Returns a source buffer with the provided text content. Copies `filename`
+  // and `text` to take ownership.
+  static auto MakeFromStringCopy(llvm::StringRef filename, llvm::StringRef text,
+                                 DiagnosticConsumer& consumer)
+      -> std::optional<SourceBuffer>;
+
   // Use one of the factory functions above to create a source buffer.
   SourceBuffer() = delete;