[dsymutil] Fix memory issue in the BinaryHolder

The BinaryHolder has two caches for object and archive entries. These
are implemented as StringMaps of ObjectEntry and ArchiveEntry
respectively. The fact that they're stored by value is problematic
because the BinaryHolder hands out references that become invalidate
when the data structure grows. This patch wraps those object instances
in unique pointers and changes the interface to hand out pointers. This
resulted in transient failures.

rdar://90412671

Differential revision: https://reviews.llvm.org/D124567