Fixed a bug in the code to create a dwarf file and directory table entires when

Fixed a bug in the code to create a dwarf file and directory table entires when
it is separating the directory part from the basename of the FileName.  Noticed
that this:

  .file 1 "dir/foo"

when assembled got the two parts switched.  Using the Mac OS X dwarfdump tool
it can be seen easily:

% dwarfdump -a a.out
include_directories[  1] = 'foo'
                Dir  Mod Time   File Len   File Name
                ---- ---------- ---------- ---------------------------
file_names[  1]    1 0x00000000 0x00000000 dir
...

Which should be:
...
include_directories[  1] = 'dir'
                Dir  Mod Time   File Len   File Name
                ---- ---------- ---------- ---------------------------
file_names[  1]    1 0x00000000 0x00000000 foo

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143521 91177308-0d34-0410-b5e6-96231b3b80d8