Driver: Move Clang "triple" computation routines to method on the

ToolChain. This fixes a potenial bad cast when running Clang on PPC code, since
the tool chain in effect is not a subclass of the Darwin one, but we were
treating it like it was.
 - This introduces some gross code duplication, but the right fix for it is to
   just move the Driver to start depending on the targets in libBasic, so I am
   not planning on fixing it immediately.

llvm-svn: 111856