Remove IfConfigDecl from the AST

The swift-syntax tree retains information about the parsed #if
regions. Drop it from the semantic AST.

lib/IRGen/GenDecl.cpp

@@ -2568,7 +2568,6 @@ void IRGenModule::emitGlobalDecl(Decl *D) {
   case DeclKind::TypeAlias:
   case DeclKind::GenericTypeParam:
   case DeclKind::AssociatedType:
-  case DeclKind::IfConfig: 
   case DeclKind::PoundDiagnostic:
   case DeclKind::Macro:
     return;
@@ -5706,7 +5705,6 @@ void IRGenModule::emitNestedTypeDecls(DeclRange members) {
     case DeclKind::Missing:
       llvm_unreachable("missing decl in IRGen");
 
-    case DeclKind::IfConfig:
     case DeclKind::PoundDiagnostic:
     case DeclKind::Macro:
       continue;