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).

testing/base/source_gen_test.cpp

@@ -147,7 +147,11 @@ auto TestCompile(llvm::StringRef source) -> bool {
       new llvm::vfs::InMemoryFileSystem;
   InstallPaths installation(
       InstallPaths::MakeForBazelRunfiles(Testing::GetExePath()));
-  Driver driver(fs, &installation, llvm::outs(), llvm::errs());
+  Driver driver({.fs = fs,
+                 .installation = &installation,
+                 .input_stream = nullptr,
+                 .output_stream = &llvm::outs(),
+                 .error_stream = &llvm::errs()});
 
   AddPreludeFilesToVfs(installation, fs);