Rename functions which only print a diagnostic to Diagnose* (#3886)

This is something I noticed working on
https://github.com/carbon-language/carbon-lang/pull/3884; I think we
have more functions named Diagnose* at present than Emit* or Report*, so
just trying to consolidate. Note a couple Diagnose* functions do a
little more validation, but maybe those should actually be renamed away
(zygoloid had mentioned wanting to generally split out diagnostics to
their own function, and then we'd have it be a more common pattern).

toolchain/parse/handle_base.cpp

@@ -12,7 +12,7 @@ auto HandleBaseDecl(Context& context) -> void {
 
   auto semi = context.ConsumeIf(Lex::TokenKind::Semi);
   if (!semi && !state.has_error) {
-    context.EmitExpectedDeclSemi(context.tokens().GetKind(state.token));
+    context.DiagnoseExpectedDeclSemi(context.tokens().GetKind(state.token));
     state.has_error = true;
   }