Switch custom error stream output to diagnostic (#4846)

This switches most error printing to use diagnostics instead of direct
stream writes, even when not a specific file diagnostic. I'm allowing
empty filenames for this use-case.

This allows a little more specific testing to validate coverage of
output using the diagnostic coverage test. I'm adding a few tests to
cover things that weren't previously tested.

Separately, this also forces a little more standardization in format...
considering how changes like #4568 show effort being spent to _mirror_
diagnostic style, my thought is now to just use diagnostic code where
possible.

Note this also allows incrementally better testing of the language
server; I'm changing the crash fix from #4847 in favor of diagnostic
testing.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/driver/clang_subcommand.cpp

@@ -51,8 +51,10 @@ auto ClangSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   // Don't run Clang when fuzzing, it is known to not be reliable under fuzzing
   // due to many unfixed issues.
   if (driver_env.fuzzing) {
-    *driver_env.error_stream
-        << "error: cannot run `clang` subcommand productively when fuzzing\n";
+    CARBON_DIAGNOSTIC(
+        ClangFuzzingDisallowed, Error,
+        "preventing fuzzing of `clang` subcommand due to library crashes");
+    driver_env.emitter.Emit(ClangFuzzingDisallowed);
     return {.success = false};
   }