[flang] Another SUBMODULE unparsing tweak, and restore buildability with gcc < 8.1

Original-commit: flang-compiler/f18@86b5db4b23
Reviewed-on: https://github.com/flang-compiler/f18/pull/130
Tree-same-pre-rewrite: false

flang/lib/parser/unparse.cc

@@ -1474,7 +1474,7 @@ public:
             }},
         x.u);
   }
-  void Before(const SubmoduleStmt &x) {  // R1417
+  void Unparse(const SubmoduleStmt &x) {  // R1417
     Word("SUBMODULE ("), WalkTupleElements(x.t, ")"), Indent();
   }
   void Unparse(const ParentIdentifier &x) {  // R1418

flang/lib/semantics/mod-file.h

@@ -15,9 +15,6 @@
 #ifndef FORTRAN_SEMANTICS_MOD_FILE_H_
 #define FORTRAN_SEMANTICS_MOD_FILE_H_
 
-#include <filesystem>
-#include <iosfwd>
-
 namespace Fortran::semantics {
 
 void WriteModFiles();