Fix two "implode()" order issues arising from wilds/experimental collapse

Summary:
Ref T13490. There's one simple "implode()" order issue here, and one slightly more complex one that uses "DIRECTORY_SEPARATOR" as glue.

Add test coverage for this, update the lint check to detect constants used as glue, and fix the callsites.

Test Plan:
  - Added a failing test and made it pass.
  - Ran `arc lint --everything` and looked for remaining implode warnings, found none.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21024

src/filesystem/Filesystem.php

@@ -1119,7 +1119,7 @@ final class Filesystem extends Phobject {
   }
 
   public static function concatenatePaths(array $components) {
-    $components = implode($components, DIRECTORY_SEPARATOR);
+    $components = implode(DIRECTORY_SEPARATOR, $components);
 
     // Replace any extra sequences of directory separators with a single
     // separator, so we don't end up with "path//to///thing.c".

src/lint/linter/xhpast/rules/ArcanistImplodeArgumentOrderXHPASTLinterRule.php

@@ -23,7 +23,17 @@ final class ArcanistImplodeArgumentOrderXHPASTLinterRule
       }
 
       $parameter = $parameters->getChildByIndex(1);
-      if (!$parameter->isStaticScalar()) {
+
+      // If the value is a static scalar, like a string literal, it's probably
+      // the glue.
+      $is_scalar = $parameter->isStaticScalar();
+
+      // If the value is a constant, like "DIRECTORY_SEPARATOR", it's probably
+      // the glue.
+      $is_constant = ($parameter->getTypeName() === 'n_SYMBOL_NAME');
+
+      $looks_like_glue = ($is_scalar || $is_constant);
+      if (!$looks_like_glue) {
         continue;
       }
 

src/lint/linter/xhpast/rules/__tests__/implode-argument-order/implode.lint-test

@@ -14,5 +14,10 @@ implode();
 implode($x);
 implode($x, $y, $z);
 
+// This should raise a warning by guessing that "DIRECTORY_SEPARATOR" is
+// glue.
+implode($path_list, DIRECTORY_SEPARATOR);
+
 ~~~~~~~~~~
 error:7:1:XHP129:Implode With Glue First
+error:19:1:XHP129:Implode With Glue First

src/workflow/ArcanistCloseWorkflow.php

@@ -90,7 +90,7 @@ EOTEXT
         "%s\n",
         pht(
           "Valid status options are:\n\t%s",
-          implode($this->getStatusOptions(), ', ')));
+          implode(', ', $this->getStatusOptions())));
       return 0;
     }
     $ids = $this->getArgument('task_id');