Add regression test for @var annotation before foreach not defining the expression variable

- Add regression test for phpstan/phpstan#6688 verifying that `@var` PHPDoc
  annotations before `foreach` correctly define the expression variable in scope
- Test covers the original issue: `/** @var string[][] $result */ foreach ($result as $data)`
  and the maintainer's reproduction: `/** @var string[] $c */ foreach ($c as $v)`
- The fix was applied in commit 4a08845 by moving the `processVarAnnotation`
  call before `processExprNode` in the `Foreach_` branch of `NodeScopeResolver`
- Investigated analogous constructs (if, while, do-while, for, switch, match):
  all are correctly handled by the general `processStmtVarAnnotation` call at
  the top of `processStmtNode` — only `Foreach_` was excluded from that general
  handling and needed its own pre-expression annotation processing

Author: ondrejmirtes

tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php

@@ -1517,4 +1517,18 @@ public function testBug11218(): void
 		$this->analyse([__DIR__ . '/data/bug-11218.php'], []);
 	}
 
+	public function testBug6688(): void
+	{
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = false;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+		$this->analyse([__DIR__ . '/data/bug-6688.php'], [
+			[
+				'Variable $a might not be defined.',
+				5,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/bug-6688.php

@@ -0,0 +1,17 @@
+<?php declare(strict_types = 1);
+
+namespace Bug6688;
+
+echo $a; // possibly undefined
+
+/** @var string $b */
+echo $b; // willed into existence
+
+/** @var string[] $c */
+foreach ($c as $v) { // willed into existence
+}
+
+/** @var string[][] $result */
+foreach ($result as $data) {
+	echo '"' . implode('", "', $data) . '"' . "\n";
+}