Move the SourceManager from the Debugger to the Target. That way it can store...

Move the SourceManager from the Debugger to the Target.  That way it can store the per-Target default Source File & Line.
Set the default Source File & line to main (if it can be found.) at startup.  Selecting the current thread & or frame resets 
the current source file & line, and "source list" as well as the breakpoint command "break set -l <NUM>" will use the 
current source file.

llvm-svn: 139323