Allow checking for invalid @main

Changing the overload resolution behavior results in the ordering that
`main` and `$main` are typechecked. If there is an error in parsing or
sema before we have typechecked these two functions, we get weird errors
about `$main` calling the main-actor `main` function being invalid. This
is because we have already set the `@main` attribute to invalid, so we
don't recognize the `$main` context as being a main entrypoint. The
error message compounds to making a confusing situation worse given that
`$main` is implicit. So by ignoring the failed `@main` attribute, we
still annotate the `$main` and `main` function as being on the main
actor, so everyone is happy in that respect.

To get the nasty behavior, you can forget to pass `-parse-as-library`
with this commit removed.

Author: etcwilde

lib/Sema/TypeCheckConcurrency.cpp

@@ -3380,7 +3380,8 @@ getActorIsolationForMainFuncDecl(FuncDecl *fnDecl) {
   if (!declContext)
     return {};
   const bool isMainDeclContext =
-      declContext->getAttrs().hasAttribute<MainTypeAttr>();
+      declContext->getAttrs().hasAttribute<MainTypeAttr>(
+          /*allow invalid*/ true);
 
   ASTContext &ctx = fnDecl->getASTContext();