Testing for #456 found some metadata that cannot be parsed by the bridge

* Add constants for the 'A' and 'j' characters in type encoding
* Basic implementation for 'A' in objc_support

The implemtation is not entirely correct, the prefix denotes
an atomic varialble and should be read/written using atomic
instructions. The prefix is currently ignored, that should be
good enough for now.

The 'j' prefix is not supported by the bridge, and is not
used in public APIs.

pyobjc-core/Modules/objc/objc-runtime-compat.h

@@ -29,6 +29,12 @@ struct PyObjC_method {
 #define _C_BYCOPY 'O'
 #define _C_BYREF 'R'
 
+/* From the clang sources...
+ *  Both are a prefix for a basic type
+ */
+#define _C_ATOMIC 'A'
+#define _C_COMPLEX 'j' /* XXX: Requires more work to support */
+
 /* These don't actually exist in the Objective-C runtime, but are used
  * by the bridge to simplify code.
  */

pyobjc-core/Modules/objc/objc_support.m

@@ -328,7 +328,7 @@ - (PyObject* _Nullable)__pyobjc_PythonTransient__:(int*)cookie
 {
     while (*type == _C_CONST || *type == _C_IN || *type == _C_INOUT || *type == _C_OUT
            || *type == _C_BYCOPY || *type == _C_BYREF || *type == _C_ONEWAY
-           || *type == 'O') {
+           || *type == 'O' || *type == _C_ATOMIC) {
         type++;
     }
     while (*type && isdigit(*type)) { // LCOV_BR_EXCL_LINE