Add vlog output to the parser. (#2435)

Might eventually want to change this further, but I'm just adding the quick framework for it.

toolchain/parser/parse_tree_fuzzer.cpp

@@ -35,7 +35,7 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
 
   // Now parse it into a tree. Note that parsing will (when asserts are enabled)
   // walk the entire tree to verify it so we don't have to do that here.
-  ParseTree::Parse(tokens, NullDiagnosticConsumer());
+  ParseTree::Parse(tokens, NullDiagnosticConsumer(), /*vlog_stream=*/nullptr);
   return 0;
 }