When parsing the DBGSourcePathRemapping plist entries

in a dSYM, and it's a version 2 DBGSourcePathRemapping,
in addition to the build/source paths specified, add 
build/source paths with the last two filename components
removed.  This more generic remapping can sometimes
help lldb to find the correct source file in complex
projects.
<rdar://problem/33973545> 

llvm-svn: 311622