Reset seen symbols when ending namespace (GH-15244)

Previously, seen symbols were never cleaned during the compilation of a single
file. This makes it impossible to use a class or function from a different
namespace if such a symbol is also declared within the same file. This is
inconsistent with how it would work when split into different files.

Author: iluuu1994

Zend/tests/use_function/ns_end_resets_seen_symbols_1.phpt

@@ -0,0 +1,58 @@
+--TEST--
+Namespace end resets seen function symbols
+--FILE--
+<?php
+
+namespace {
+    function f() {
+        echo __FUNCTION__, "\n";
+    }
+    f();
+}
+
+namespace Ns {
+    function f() {
+        echo __FUNCTION__, "\n";
+    }
+    f();
+}
+
+namespace {
+    use function Ns\f;
+    f();
+}
+
+namespace Ns {
+    use function f;
+    f();
+}
+
+namespace {
+    f();
+}
+
+namespace Ns {
+    f();
+}
+
+namespace {
+    use function f;
+    f();
+}
+
+namespace Ns {
+    use function Ns\f;
+    f();
+}
+
+?>
+--EXPECTF--
+Warning: The use statement with non-compound name 'f' has no effect in %s on line 36
+f
+Ns\f
+Ns\f
+f
+f
+Ns\f
+f
+Ns\f

Zend/tests/use_function/ns_end_resets_seen_symbols_2.phpt

@@ -0,0 +1,62 @@
+--TEST--
+Namespace end resets seen class symbols
+--FILE--
+<?php
+
+namespace {
+    class C {}
+    var_dump(new C);
+}
+
+namespace Ns {
+    class C {}
+    var_dump(new C);
+}
+
+namespace {
+    use Ns\C;
+    var_dump(new C);
+}
+
+namespace Ns {
+    use C;
+    var_dump(new C);
+}
+
+namespace {
+    var_dump(new C);
+}
+
+namespace Ns {
+    var_dump(new C);
+}
+
+namespace {
+    use C;
+    var_dump(new C);
+}
+
+namespace Ns {
+    use Ns\C;
+    var_dump(new C);
+}
+
+?>
+--EXPECTF--
+Warning: The use statement with non-compound name 'C' has no effect in %s on line 32
+object(C)#%d (0) {
+}
+object(Ns\C)#1 (0) {
+}
+object(Ns\C)#1 (0) {
+}
+object(C)#%d (0) {
+}
+object(C)#%d (0) {
+}
+object(Ns\C)#1 (0) {
+}
+object(C)#%d (0) {
+}
+object(Ns\C)#1 (0) {
+}

Zend/zend_compile.c

@@ -377,6 +377,8 @@ static void zend_reset_import_tables(void) /* {{{ */
 		efree(FC(imports_const));
 		FC(imports_const) = NULL;
 	}
+
+	zend_hash_clean(&FC(seen_symbols));
 }
 /* }}} */