Fix crash with async main and no conc module

The concurrency module gets imported implicitly when
`-enable-experimental-concurrency` is passed to the compiler. If the
concurrency module isn't imported, we crash on th assert verifying that
we were able to find it. We need the module to lookup the
`_runAsyncMain` function. This patch updates the compiler to stop
type-checking the async-main function and emit an error if the module
isn't available instead of crashing.

Author: etcwilde

include/swift/AST/DiagnosticsSema.def

@@ -4379,6 +4379,9 @@ ERROR(actorindependent_local_var,none,
 ERROR(concurrency_lib_missing,none,
       "missing '%0' declaration, probably because the '_Concurrency' "
       "module was not imported", (StringRef))
+ERROR(async_main_no_concurrency,none,
+      "'_Concurrency' module not imported, required for async main", ())
+
 ERROR(enqueue_partial_task_not_in_context,none,
       "'enqueue(partialTask:)' can only be implemented in the definition of "
       "actor class %0", (Type))

lib/Sema/TypeCheckAttr.cpp

@@ -1835,7 +1835,17 @@ synthesizeMainBody(AbstractFunctionDecl *fn, void *arg) {
     // Resulting $main looks like:
     // $main() { _runAsyncMain(main) }
     auto *concurrencyModule = context.getLoadedModule(context.Id_Concurrency);
-    assert(concurrencyModule != nullptr && "Failed to find Concurrency module");
+    if (!concurrencyModule) {
+      context.Diags.diagnose(mainFunction->getAsyncLoc(),
+                             diag::async_main_no_concurrency);
+      auto result = new (context) ErrorExpr(mainFunction->getSourceRange());
+      SmallVector<ASTNode, 1> stmts;
+      stmts.push_back(result);
+      auto body = BraceStmt::create(context, SourceLoc(), stmts,
+                                    SourceLoc(), /*Implicit*/true);
+
+      return std::make_pair(body, /*typechecked*/true);
+    }
 
     SmallVector<ValueDecl *, 1> decls;
     concurrencyModule->lookupQualified(