Add methods to get archive file name from member file.

Previously we only have File::path() to get the path name of a file.
If a file was a member of an archive file, path() returns a concatenated
string of the file name in the archive and the archive file name.
If we wanted to get a file name or an archive file name, we had to
parse that string. That's of course not good.

This patch adds new member functions, archivePath and memberPath, to File.

http://reviews.llvm.org/D7447

llvm-svn: 228352