Reland r338088, "ELF: Make --print-icf-sections output deterministic."

The xxHash64 function has been made unsigned-char-independent, so
we can reland this change now.

Original commit message:
> 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: 338153