Move the code from 'llvmc/driver' into a new CompilerDriver library, and change
the build system accordingly. Makes it easier for projects using LLVM to build
their own llvmc-based drivers.
Tested with objdir != srcdir.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65821 91177308-0d34-0410-b5e6-96231b3b80d8
