Add -fobjc-default-synthesized-properties flag

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122519 91177308-0d34-0410-b5e6-96231b3b80d8