Pre-compute the tail of the archive

An archive looks like

<header>
<symbol table>
<tail>

The symbol table refers to offsets in the tail. A complication is that
we would like to support symbol tables that use 64 bit offsets if it
turns out that any of the offsets is too big.

This patch changes the archive writer to first compute the tail. We
cannot just compute one big StringRef since that would require reading
every member upfront, but we can represent it as a series of
StringRefs.

Having done that it is much easier to compute the symbol table and all
offsets are computed before it is written. With this if there is an
accounting problem it will show up with a regular symbol table, not
just when a 64 bit one is needed.

llvm-svn: 314844