Improve handling of .file, .include and .incbin directives to
allow escaped octal character sequences.

The patch was discussed in Phabricator. See:
http://llvm-reviews.chandlerc.com/D1289



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190089 91177308-0d34-0410-b5e6-96231b3b80d8