[Support/TarWriter] - Don't allow TarWriter to add the same file more than once.

This is for PR35460.

Currently when LLD adds files to TarWriter it may pass the same file
multiple times. For example it happens for clang reproduce file which specifies
archive (.a) files more than once in command line. 
Patch makes TarWriter to ignore files with the same path, so it will
add only the first one to archive.

Differential revision: https://reviews.llvm.org/D40606

llvm-svn: 319750