[COFF] Fix wholearchive with thin archives

The Archive object created when loading an archive specified with
wholearchive got cleaned up immediately, when the owning std::unique_ptr
went out of scope, even if persisted StringRefs pointed to memory that
belonged to the archive, which no longer was mapped in memory.

This hasn't been an issue with regular (as opposed to thin) archives,
as references to the member objects has kept the mapping for the whole
archive file alive - but with thin archives, all such references point
to other files.

Add the std::unique_ptr to the arena allocator, to retain it as long
as necessary.

This fixes (the last issue raised in) PR42388.

Differential Revision: https://reviews.llvm.org/D65565

llvm-svn: 367599