rdar://problem/10227672

There were two problems associated with this radar:
1. "settings show target.source-map" failed to show the source-map after, for example,
   "settings set target.source-map /Volumes/data/lldb/svn/trunk/test/source-manager /Volumes/data/lldb/svn/trunk/test/source-manager/hidden"
   has been executed to set the source-map.
2. "list -n main" failed to display the source of the main() function after we properly set the source-map.

The first was fixed by adding the missing functionality to TargetInstanceSettings::GetInstanceSettingsValue (Target.cpp)
and updating the support files PathMappingList.h/.cpp; the second by modifying SourceManager.cpp to fix several places
with incorrect logic.

Also added a test case test_move_and_then_display_source() to TestSourceManager.py, which moves main.c to hidden/main.c,
sets target.source-map to perform the directory mapping, and then verifies that "list -n main" can still show the main()
function.

llvm-svn: 146422