More property attribute recovery improvements. Instead of this:

More property attribute recovery improvements.  Instead of this:

crash.m:8:12: error: type name requires a specifier or qualifier
@property (readonlyx, getter=isAwesome) int _awesome;
           ^
crash.m:8:29: error: expected ';' at end of declaration list
@property (readonlyx, getter=isAwesome) int _awesome;
                            ^
crash.m:8:39: error: expected identifier or '('
@property (readonlyx, getter=isAwesome) int _awesome;
                                      ^

we now get:

crash.m:8:12: error: unknown property attribute 'readonlyx'
@property (readonlyx, getter=isAwesome) int _awesome;
           ^

Also, we can eliminate isObjCPropertyAttribute now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57811 91177308-0d34-0410-b5e6-96231b3b80d8