ELF: Make --print-icf-sections output deterministic.

The icf-safe.s test currently fails on 32-bit platforms because it uses
the --print-icf-sections flag and depends on the output appearing in
a specific order. However, this flag causes the output to depend on
the order of the sections in the Sections array, which depends on the
hash values returned from hash_combine, which happen to be different
for that test between 32-bit and 64-bit platforms.

This change makes the output deterministic by using xxHash64 instead of
hash_combine.

Differential Revision: https://reviews.llvm.org/D49877

llvm-svn: 338088