Don't forward @main into SIL

We've already generated the parts necessary to handle `@main` during
SILGen. Keeping the attribute on the struct means that re-ingesting the
SIL will fail because it tries to re-create the implicit `$main`
function resulting in the following error message:

error: invalid redeclaration of synthesized static method '$main()'
  static func $main()

Author: etcwilde

lib/AST/Attr.cpp

@@ -895,9 +895,13 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     }
     return true;
 
-  case DAK_MainType:
+  case DAK_MainType: {
+    // Don't print into SIL. Necessary bits have already been generated.
+    if (Options.PrintForSIL)
+      return false;
     Printer.printSimpleAttr(getAttrName(), /*needAt=*/true);
     return true;
+  }
 
   case DAK_SetterAccess:
     Printer.printKeyword(getAttrName(), Options, "(set)");

test/attr/ApplicationMain/attr_main_struct.swift

@@ -1,7 +1,11 @@
 // RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s
+// RUN: %target-swift-frontend -emit-silgen -parse-as-library %s | %FileCheck %s
 
 @main
 struct EntryPoint {
   static func main() {
   }
 }
+
+// CHECK-NOT: @main struct EntryPoint {
+// CHECK: struct EntryPoint {