Fix a bug in "target.source-map" where we would resolve unmapped paths incorrectly

When using:

(lldb) settings set target.source-map ./ /path/to/source
LLDB would fail to set a source file and line breakpoint with:

(lldb) breakpoint set --file /path/to/source/main.c --line 2
Because code in the target was undoing the remapping of "/path/to/source/main.c" to "./main.c" and then it would resolve this path, which would append the current working directory to the path. We don't want to resolve paths that we unmap.

Test case added.

Differential Revision: https://reviews.llvm.org/D44502

llvm-svn: 327600