Refactor many file functions to use FileSpec over strings.

Summary:
This should solve the issue of sending denormalized paths over gdb-remote
if we stick to GetPath(false) in GDBRemoteCommunicationClient, and let the
server handle any denormalization.

Reviewers: ovyalov, zturner, vharron, clayborg

Reviewed By: clayborg

Subscribers: tberghammer, emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D9728

llvm-svn: 238604