Add -fobjc-default-synthesized-properties flag

to allow us to explicitly control whether or
not Objective-C properties are default synthesized.
Currently this feature only works when using
the -fobjc-non-fragile-abi2 flag (so there is
no functionality change), but we can now turn
off this feature without turning off all the features
coupled with -fobjc-non-fragile-abi2.

llvm-svn: 122519