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/source/BUILD

@@ -13,6 +13,7 @@ cc_library(
     deps = [
         "//common:error",
         "//toolchain/diagnostics:diagnostic_emitter",
+        "//toolchain/diagnostics:file_diagnostics",
         "//toolchain/diagnostics:format_providers",
         "@llvm-project//llvm:Support",
     ],

toolchain/source/source_buffer.cpp

@@ -7,18 +7,10 @@
 #include <limits>
 
 #include "llvm/Support/ErrorOr.h"
+#include "toolchain/diagnostics/file_diagnostics.h"
 
 namespace Carbon {
 
-namespace {
-struct FilenameConverter : DiagnosticConverter<llvm::StringRef> {
-  auto ConvertLoc(llvm::StringRef filename, ContextFnT /*context_fn*/) const
-      -> ConvertedDiagnosticLoc override {
-    return {.loc = {.filename = filename}, .last_byte_offset = -1};
-  }
-};
-}  // namespace
-
 auto SourceBuffer::MakeFromStdin(DiagnosticConsumer& consumer)
     -> std::optional<SourceBuffer> {
   return MakeFromMemoryBuffer(llvm::MemoryBuffer::getSTDIN(), "<stdin>",
@@ -29,8 +21,7 @@ auto SourceBuffer::MakeFromFile(llvm::vfs::FileSystem& fs,
                                 llvm::StringRef filename,
                                 DiagnosticConsumer& consumer)
     -> std::optional<SourceBuffer> {
-  FilenameConverter converter;
-  DiagnosticEmitter<llvm::StringRef> emitter(converter, consumer);
+  FileDiagnosticEmitter emitter(&consumer);
 
   llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> file =
       fs.openFileForRead(filename);
@@ -64,8 +55,7 @@ auto SourceBuffer::MakeFromMemoryBuffer(
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> buffer,
     llvm::StringRef filename, bool is_regular_file,
     DiagnosticConsumer& consumer) -> std::optional<SourceBuffer> {
-  FilenameConverter converter;
-  DiagnosticEmitter<llvm::StringRef> emitter(converter, consumer);
+  FileDiagnosticEmitter emitter(&consumer);
 
   if (buffer.getError()) {
     CARBON_DIAGNOSTIC(ErrorReadingFile, Error, "error reading file: {0}",