[llvm-ar] Implement the P modifier.

Summary:
GNU ar has a `P` modifier that changes filename comparisons to use full paths instead of the basename. As noted in the GNU docs, regular archives are not created with full path names, so P is used to deal with archives created by other archive programs (e.g. see the updated `absolute-paths.test` test case).

Since thin archives use full path names -- paths are relative to the archive -- it seems very error prone to not imply P when dealing with thin archives, so P is implied in those cases. (I think this is a deviation from GNU ar that makes sense).

This fixes PR37436 via https://github.com/ClangBuiltLinux/linux/issues/33.

Reviewers: mstorsjo, pcc, ruiu, davide, david2050, rnk

Subscribers: tpimh, llvm-commits, nickdesaulniers

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57927

llvm-svn: 354044