[BPF] [BTF] Process FileName with absolute path correctly

In IR, sometimes the following attributes for DIFile may be
generated:
  filename: /home/yhs/test.c
  directory: /tmp
The /tmp may represent the working directory of the compilation
process.

In such cases, since filename is with absolute path,
the directory should be ignored by BTF. The filename alone is
enough to get the source.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 352952