[ELF] Skip over empty sections when checking for contiguous relro

When checking for contiguous relro sections we can skip over empty sections.
If there is an empty non-relro section in the middle of a contiguous block
of relro sections then it cannot be written to so it is safe to include in
PT_GNU_RELRO header. If there is a contiguous block of empty relro sections
then no PT_GNU_RELRO header is required for them.

Differential Revision: https://reviews.llvm.org/D40364

llvm-svn: 318924