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/diagnostics/coverage_test.cpp

@@ -24,8 +24,10 @@ constexpr DiagnosticKind UntestedDiagnosticKinds[] = {
     DiagnosticKind::TestDiagnostic,
     DiagnosticKind::TestDiagnosticNote,
 
-    // Driver specific.
-    DiagnosticKind::CppInteropFuzzing,
+    // Diagnosing erroneous install conditions, but test environments are
+    // typically correct.
+    DiagnosticKind::CompilePreludeManifestError,
+    DiagnosticKind::DriverInstallInvalid,
 
     // These diagnose filesystem issues that are hard to unit test.
     DiagnosticKind::ErrorReadingFile,