Switch Driver back to parameters for construction (#4849)

This is for more complex construction, see #4846

testing/base/source_gen_test.cpp

@@ -147,11 +147,8 @@ auto TestCompile(llvm::StringRef source) -> bool {
       new llvm::vfs::InMemoryFileSystem;
   InstallPaths installation(
       InstallPaths::MakeForBazelRunfiles(Testing::GetExePath()));
-  Driver driver({.fs = fs,
-                 .installation = &installation,
-                 .input_stream = nullptr,
-                 .output_stream = &llvm::outs(),
-                 .error_stream = &llvm::errs()});
+  Driver driver(fs, &installation, /*input_stream=*/nullptr, &llvm::outs(),
+                &llvm::errs());
 
   AddPreludeFilesToVfs(installation, fs);