Refactor diagnostics out of parser_context.h (#2834)

This is addressing an issue left behind by the context switch, removing a few diagnostics that had been in the header rather than figuring out proper homes. I'm splitting one for semis a little further, sharing one, and then the other two are actually able to be moved into more specific homes as-is (one is only used in one place, clearly an oversight that it wasn't there already).

toolchain/parser/parser_handle_declaration_name_and_params.cpp

@@ -13,6 +13,9 @@ static auto ParserHandleDeclarationNameAndParams(ParserContext& context,
 
   if (!context.ConsumeAndAddLeafNodeIf(TokenKind::Identifier,
                                        ParseNodeKind::DeclaredName)) {
+    CARBON_DIAGNOSTIC(ExpectedDeclarationName, Error,
+                      "`{0}` introducer should be followed by a name.",
+                      TokenKind);
     context.emitter().Emit(*context.position(), ExpectedDeclarationName,
                            context.tokens().GetKind(state.token));
     context.ReturnErrorOnState();