Add a way for diagnostics to sort on more than last_byte_offset. (#6687)

The intent is that `last_byte_offset` is still the main sorting key.
Diagnostics issued normally (e.g. in an expression) will keep sorting
the same, and come before the new diagnostic sort. Diagnostics issued at
the end of a scope (e.g. `unused`) can request sorting by their start
location, and would become interleaved through that.

Choosing "on scope" because I think that's the main way we'll use this
functionality (on scope changes); can always rename later if usage
expands.

Assisted-by: Google Antigravity with Gemini 3 Flash

toolchain/diagnostics/coverage_test.cpp

@@ -22,6 +22,7 @@ constexpr Kind Kinds[] = {
 constexpr Kind UntestedKinds[] = {
     // These exist only for unit tests.
     Kind::TestDiagnostic,
+    Kind::TestDiagnosticOnScope,
     Kind::TestDiagnosticNote,
 
     // Diagnosing erroneous install conditions, but test environments are