Break out driver environment info into its own type. (#4299)

I want to split commands out so that we don't keep piling onto driver
(particularly as I'm eyeing clang-related commands). This extracts out
the DriverEnv so that it can be easily shared, with the CompilationUnit
as an example.

CARBON_VLOG_TO is to remove the vlog_stream_ requirement of CARBON_VLOG.

common/vlog_test.cpp

@@ -49,5 +49,13 @@ TEST(VLogTest, Disabled) {
   EXPECT_THAT(vlog.TakeStr(), IsEmpty());
 }
 
+TEST(VLogTest, To) {
+  TestRawOstream buffer;
+  CARBON_VLOG_TO(&buffer, "Test");
+  EXPECT_THAT(buffer.TakeStr(), "Test");
+}
+
+TEST(VLogTest, ToNull) { CARBON_VLOG_TO(nullptr, "Unused"); }
+
 }  // namespace
 }  // namespace Carbon::Testing