[llvm-symbolizer] Add support for --basenames/-s

This fixes https://bugs.llvm.org/show_bug.cgi?id=40068.

--basenames is a GNU addr2line switch which strips the directory names
from the file path in the output.

Reviewed by: ruiu

Differential Revision: https://reviews.llvm.org/D56919

llvm-svn: 351795