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_function.cpp

@@ -74,9 +74,7 @@ auto ParserHandleFunctionSignatureFinish(ParserContext& context) -> void {
     }
     default: {
       if (!state.has_error) {
-        context.emitter().Emit(*context.position(),
-                               ExpectedDeclarationSemiOrDefinition,
-                               TokenKind::Fn);
+        context.EmitExpectedDeclarationSemiOrDefinition(TokenKind::Fn);
       }
       // Only need to skip if we've not already found a new line.
       bool skip_past_likely_end =