Give same-named members unique timestamps on Darwin in llvm-ar.

This change ensures that the (membername,timestamp) tuple uniquely
identifies an entry in an archive for format=darwin, in deterministic
mode (which is the default).

That, then, enables lldb and dsymutil to locate the appropriate object
within the archive.

Differential Revision: https://reviews.llvm.org/D47659

llvm-svn: 343805