Make .file directive to have basename only

Summary:
Currently LLVM puts directory along with the filename in .file directive, but this behavior doesn't match gcc. There's a no clear description about which one is right (https://sourceware.org/binutils/docs/as/File.html#File), but one document (https://sourceware.org/gdb/current/onlinedocs/stabs/ELF-Linker-Relocation.html) suggests that STT_FILE symbol in elf file is expected to have basename only, which should have a same sting file .file directive according to (https://docs.oracle.com/cd/E26502_01/html/E28388/eoiyg.html).

This also affects badly on the build system that uses hashing, as the directory info could be differnt from developer to developer even when they're working on same file.

Reviewers: pcc, mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36018

llvm-svn: 310642