Add functionality to the platforms to figure out the proper name for a dynamic...

Add functionality to the platforms to figure out the proper name for a dynamic library on the system given a basename
This will do things like,
given mylibrary,
return

libmylibrary.dylib on OSX
mylibrary.dll on Windows

and so on for other platforms
It is currently implemented for Windows, Darwin, and Linux. Other platforms should fill in accordingly

llvm-svn: 246131