gen_stub: Fix handling of escape sequences in generated C strings

When handling sequences like this in a stub:

```php
<?php
class Whatever {
    public static string $foobar1 = "CCC \n\r\t\v\e\f\\\$\"\101\x41\u{41} CCC";
    public static string $foobar2 = 'CCC \n\r\t\v\e\f\\\$\"\101\x41\u{41} CCC';
}
```

...properly generate C headers that properly escape the string.
Otherwise, the differing escaping rules and differences between PHP's
single and double quoted strings could lead to mangled headers.

The output of these strings after the stub has been generated:

```
string(20) "CCC

        $"AAA CCC"
string(41) "CCC \n\r\t\v\e\f\\\\$\"\101\x41\u{41} CCC"
```

And the generated arginfo:

```c
        zval property_foobar1_default_value;
        zend_string *property_foobar1_default_value_str = zend_string_init("CCC \n\r\t\v\x1b\f\\\$\"AAA CCC", strlen("CCC \n\r\t\v\x1b\f\\\$\"AAA CCC"), 1);
        ZVAL_STR(&property_foobar1_default_value, property_foobar1_default_value_str);
        zend_string *property_foobar1_name = zend_string_init("foobar1", sizeof("foobar1") - 1, true);
        zend_declare_typed_property(class_entry, property_foobar1_name, &property_foobar1_default_value, ZEND_ACC_PUBLIC|ZEND_ACC_STATIC, NULL, (zend_type) ZEND_TYPE_INIT_MASK(MAY_BE_STRING));
        zend_string_release_ex(property_foobar1_name, true);

        zval property_foobar2_default_value;
        zend_string *property_foobar2_default_value_str = zend_string_init("CCC \\n\\r\\t\\v\\e\\f\\\\\\\\$\\\"\\101\\x41\\u{41} CCC", strlen("CCC \\n\\r\\t\\v\\e\\f\\\\\\\\$\\\"\\101\\x41\\u{41} CCC"), 1);
        ZVAL_STR(&property_foobar2_default_value, property_foobar2_default_value_str);
        zend_string *property_foobar2_name = zend_string_init("foobar2", sizeof("foobar2") - 1, true);
        zend_declare_typed_property(class_entry, property_foobar2_name, &property_foobar2_default_value, ZEND_ACC_PUBLIC|ZEND_ACC_STATIC, NULL, (zend_type) ZEND_TYPE_INIT_MASK(MAY_BE_STRING));
        zend_string_release_ex(property_foobar2_name, true);

```

Note that the PHP escape sequence "\$" will generate a warning with
clang at least, as C doesn't have an equivalent:

```
/Users/calvin/src/php-src/ext/zend_test/test_arginfo.h:799:90: warning: unknown escape sequence '\$' [-Wunknown-escape-sequence]
[...]
```

I'm also unsure how best to make tests for gen_stub, so I haven't
included any beyond mentioning how to test manually with this commit.

Fixes GH-22169.

Author: NattyNarwhal

build/gen_stub.php

@@ -2344,9 +2344,21 @@ public function getCExpr(): ?string
         // $this->expr has all its PHP constants replaced by C constants
         $prettyPrinter = new Standard;
         $expr = $prettyPrinter->prettyPrintExpr($this->expr);
-        // PHP single-quote to C double-quote string
         if ($this->type->isString()) {
-            $expr = preg_replace("/(^'|'$)/", '"', $expr);
+            // The string in $expr has had the octal, hex, and unicode
+            // backslash sequences already applied for double-quoted strings,
+            // but not the other sequences.
+            // PHP has single quote strings, C doesn't (they're one char).
+            // Single-quoted strings need handling to replace their escapes
+            // with the double-quoted equivalent; namely single quote escapes.
+            // Double-quoted strings have similar escape sequences as C does,
+            // so we can pass them through directly.
+            if (preg_match("/(^'|'$)/", $expr)) {
+                $expr = substr($expr, 1, -1); // strip quotes, readd later
+                $expr = str_replace("\\'", "'", $expr);
+                $expr = addcslashes($expr, "\\\"");
+                $expr = "\"$expr\"";
+            }
         }
         return $expr[0] == '"' ? $expr : preg_replace('(\bnull\b)', 'NULL', str_replace('\\', '', $expr));
     }