Add stdin to driver's streams, and refactor stream passing (#4812)

The language server needs stdin, and for tests we should be passing it
around. My intent is to pass in a faux stdin to Driver for language
server tests.

As long as I'm adding a new parameter, I was looking at also changing
the way streams are passed in to Driver for style (pointers since
they're held past construction lifetime). Since these are all stored in
DriverEnv, I thought it might be a net improvement to use the struct
directly, getting more explicit parameter names and also removing the
need for `SetFuzzing`.

I'm trying here to avoid functional changes, but there are a couple
additional fixes like removing an obsolete `find_insensitive` and
refactoring how `ValidateOptions` handles errors (because it reduces the
number of spots that operate on error_stream).

toolchain/language_server/language_server.cpp

@@ -13,14 +13,15 @@
 
 namespace Carbon::LanguageServer {
 
-auto Run(std::FILE* input_stream, llvm::raw_ostream& output_stream)
-    -> ErrorOr<Success> {
+auto Run(FILE* input_stream, llvm::raw_ostream& output_stream,
+         llvm::raw_ostream& /*error_stream*/) -> ErrorOr<Success> {
   // Set up the connection.
   std::unique_ptr<clang::clangd::Transport> transport(
       clang::clangd::newJSONTransport(input_stream, output_stream,
                                       /*InMirror=*/nullptr,
                                       /*Pretty=*/true));
   Context context;
+  // TODO: Use error_stream in IncomingMessages to report dropped errors.
   IncomingMessages incoming(transport.get(), &context);
   OutgoingMessages outgoing(transport.get());