[llvm][ADT] Fix Arm 32 bit compilation warning in lazy atomic pointer

LazyAtomicPointer.h:36:49: warning: implicit conversion from 'unsigned long long'
to 'uintptr_t' (aka 'unsigned int') changes value from 18446744073709551615 to 4294967295 [-Wconstant-conversion]
  static constexpr uintptr_t getBusy() { return -1ULL; }

On 32 bit Arm ULL is an unsigned long long which is 8 bytes, but
uintptr_t is 4 bytes. Instead of using a value, use the macro
UINTPTR_MAX that will be the correctly sized value.