Bump the value of the __APPLE_CC__ predefined macro up to 6000.

The previous value was set to match some ancient version of Apple's GCC.
The value should be higher than anything used by Apple's GCC, but we don't
intend for this value to be updated in the future. We have other macros to
identify compiler versions. <rdar://problem/14749599>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188700 91177308-0d34-0410-b5e6-96231b3b80d8