Use TI.hasBuiltinAtomic() when setting ATOMIC_*_LOCK_FREE values. NFCI

I noticed that the values for __{CLANG,GCC}_ATOMIC_POINTER_LOCK_FREE were
incorrectly set to 1 instead of two in downstream CHERI targets because
pointers are handled specially there. While fixing this downstream, I
noticed that the existing code could be refactored to use
TargetInfo::hasBuiltinAtomic instead of repeating the almost identical
logic. In theory there could be a difference here since hasBuiltinAtomic() also
returns true for types less than 1 char in size, but since
InitializePredefinedMacros() never passes such a value this change should
not introduce any functional changes.

Reviewed By: rprichard, efriedma

Differential Revision: https://reviews.llvm.org/D135142