Rename `FileSpec::IsRelativeToCurrentWorkingDirectory` to `IsRelative`.

Summary:
`IsRelativeToCurrentWorkingDirectory` was misleading, because relative paths
are sometimes appended to other directories, not just the cwd. Plus, the new
name is shorter. Also added `IsAbsolute` for completeness.

Reviewers: clayborg, ovyalov

Reviewed By: ovyalov

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D10262

llvm-svn: 239419