Description:
------------
In line with other FFI implementations, defining builtin types such as va_args, __builtin_va_list and __gnuc_va_list should not raise a redefinition error, but should be silently ignored.

For example, LUAJIT's FFI documentation (https://luajit.org/ext_ffi_semantics.html) contains the following lines:

The following C types are pre-defined by the C parser (like a typedef, except re-declarations will be ignored):

Vararg handling: va_list, __builtin_va_list, __gnuc_va_list.
From <stddef.h>: ptrdiff_t, size_t, wchar_t.
From <stdint.h>: int8_t, int16_t, int32_t, int64_t, uint8_t, uint16_t, uint32_t, uint64_t, intptr_t, uintptr_t.

The problem is that hard erroring on these types means users of FFI who want to include an entire library's header files need to manually be careful to not include these definitions.

Additionally, the error message raised is incorrect, as it's not a syntax error but instead the redefinition of a known type.

Test script:
---------------
<?php 

$cdef = '
typedef __builtin_va_list __gnuc_va_list;
';

$ffi = FFI::cdef($cdef, "/lib/x86_64-linux-gnu/libc.so.6");

Expected result:
----------------
valid FFI instance

Actual result:
--------------
Fatal error: Uncaught FFI\ParserException: unexpected '<ID>' at line 2 in test.php:7
Stack trace:
#0 test.php(7): FFI::cdef('\ntypedef __buil...', '/lib/x86_64-lin...')
#1 {main}
  thrown in test.php on line 7