Tracking modules down when you have a UUID and a path has been improved.

DynamicLoaderDarwinKernel no longer parses mach-o load commands and it
now uses the memory based modules now that we can load modules from memory.

Added a target setting named "target.exec-search-paths" which can be used
to supply a list of directories to use when trying to look for executables.
This allows one or more directories to be used when searching for modules
that may not exist in the SDK/PDK. The target automatically adds the directory
for the main executable to this list so this should help us in tracking down
shared libraries and other binaries. 

llvm-svn: 150426