Fix -break-insert not working when using absolute paths (MI)

Summary:
When trying to parse the -break-insert arguments as a named location, the string parsing was not configured to allow directory paths. This patch adds a constructor to allow the parsing of string as directory path along with the other parameters.

This fixes https://llvm.org/bugs/show_bug.cgi?id=28709

Patch from malaperle@gmail.com
Reviewers: clayborg, ki.stfu
Subscribers: lldb-commits, ki.stfu
Differential Revision: https://reviews.llvm.org/D22902

llvm-svn: 277117