Fix support for distinguishing archive members by timestamp on Darwin.

On Darwin, the binary's symbol table points to debug info in object
files -- potentially object files within a static library. Such a
library may have multiple entries with the same name, distinguished
only by timestamp.

The code was already _attempting_ to handle this case (see the code in
ObjectContainerBSDArchive::Archive::FindObject which disambiguates via
timestamp). But, unfortunately, while the timestamp was taken into
account on the _first_ lookup, the result was then cached in a map
keyed only off of the path.

Added the timestamp to the cache, and added a test case.

Differential Revision: https://reviews.llvm.org/D47660

llvm-svn: 333813