Revised comment and code style

Author: mxswd

include/swift/Option/FrontendOptions.td

@@ -228,7 +228,7 @@ def dump_clang_diagnostics : Flag<["-"], "dump-clang-diagnostics">,
   HelpText<"Dump Clang diagnostics to stderr">;
 
 def disable_modules_validate_system_headers : Flag<["-"], "disable-modules-validate-system-headers">,
-  HelpText<"Disable modules validate system headers in the clang importer">;
+  HelpText<"Disable validating system headers in the Clang importer">;
 
 def emit_verbose_sil : Flag<["-"], "emit-verbose-sil">,
   HelpText<"Emit locations during SIL emission">;

lib/ClangImporter/ClangImporter.cpp

@@ -344,10 +344,6 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
           SHIMS_INCLUDE_FLAG, searchPathOpts.RuntimeResourcePath,
       });
 
-  if (!importerOpts.DisableModulesValidateSystemHeaders) {
-    invocationArgStrs.push_back("-fmodules-validate-system-headers");
-  }
-
   // Set C language options.
   if (triple.isOSDarwin()) {
     invocationArgStrs.insert(invocationArgStrs.end(), {
@@ -487,6 +483,10 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
     invocationArgStrs.back().append(moduleCachePath);
   }
 
+  if (!importerOpts.DisableModulesValidateSystemHeaders) {
+    invocationArgStrs.push_back("-fmodules-validate-system-headers");
+  }
+
   if (importerOpts.DetailedPreprocessingRecord) {
     invocationArgStrs.insert(invocationArgStrs.end(), {
       "-Xclang", "-detailed-preprocessing-record",