Combine DiagnosticConverter into DiagnosticEmitter (#4878)

At present, we typically define a DiagnosticConverter, then store an
instance of it and a DiagnosticEmitter that wraps it. This is relatively
minor in general, but I've been trying to create more self-contained
DiagnosticEmitter classes (which hold their own DiagnosticConverter,
similar to NullDiagnosticEmitter), and there it just gets in the way.

Since we don't reuse DiagnosticConverter instances, this combines the
definition into DiagnosticEmitter. Mainly this means we don't have a
separate object in play, and less to carry around.

The most impact is probably to SemIRDiagnosticConverter, which was also
the most complex. Now `SemIRLocDiagnosticEmitter`, this gets some
different construction flow. Note in the PR I've split the file rename
to its own commit, to try to help delta views. However, the most
substantial parts of the refactoring are split into #4876, which this
depends upon.

toolchain/lex/tokenized_buffer.cpp

@@ -415,12 +415,6 @@ auto TokenizedBuffer::SourcePointerToDiagnosticLoc(const char* loc) const
           .last_byte_offset = offset};
 }
 
-auto TokenizedBuffer::SourceBufferDiagnosticConverter::ConvertLoc(
-    const char* loc, ContextFnT /*context_fn*/) const
-    -> ConvertedDiagnosticLoc {
-  return tokens_->SourcePointerToDiagnosticLoc(loc);
-}
-
 auto TokenizedBuffer::TokenToDiagnosticLoc(TokenIndex token) const
     -> ConvertedDiagnosticLoc {
   // Map the token location into a position within the source buffer.
@@ -435,10 +429,4 @@ auto TokenizedBuffer::TokenToDiagnosticLoc(TokenIndex token) const
   return converted;
 }
 
-auto TokenDiagnosticConverter::ConvertLoc(TokenIndex token,
-                                          ContextFnT /*context_fn*/) const
-    -> ConvertedDiagnosticLoc {
-  return tokens_->TokenToDiagnosticLoc(token);
-}
-
 }  // namespace Carbon::Lex