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/compile_subcommand.h

@@ -52,7 +52,6 @@ struct CompileOptions {
   bool dump_asm = false;
   bool dump_mem_usage = false;
   bool dump_timings = false;
-  bool include_diagnostic_kind = false;
   bool stream_errors = false;
   bool preorder_parse_tree = false;
   bool builtin_sem_ir = false;
@@ -75,8 +74,9 @@ class CompileSubcommand : public DriverSubcommand {
 
  private:
   // Does custom validation of the compile-subcommand options structure beyond
-  // what the command line parsing library supports.
-  auto ValidateOptions() const -> ErrorOr<Success>;
+  // what the command line parsing library supports. Diagnoses and returns false
+  // on failure.
+  auto ValidateOptions(NoLocDiagnosticEmitter& emitter) const -> bool;
 
   CompileOptions options_;
 };