[ELF] Make InputSection smaller

On LP64/Windows platforms, this decreases sizeof(InputSection) from 208 (larger
on Windows) to 184.

For a large executable (7.6GiB, inputSections.size()=5105122,
make<InputSection> called 4835760 times), this decreases cgroup
memory.max_usage_in_bytes by 0.6%

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D91018