[libc][hdrgen] Fix `includes` sorting in JSON emission (#165460)

The JSON output support in hdrgen had a bug that tripped when
used with headers that use special-case headers like <stdint.h>
to supply some times, as well as llvm-libc-types/*.h headers.

GitOrigin-RevId: 10afda0796cf543390fa5670d757b3ff7a558224