llvm-config: Start stubbing out a C++ version of llvm-config (as llvm-config-2,

for now).

 - Mostly complete except for the data that needs to come from the build system
   or the dependencies.

 - Has some small improvements from current llvm-config:

   o Uses TargetRegistry directly to get some information.

   o Designed to work correctly when used from a CMake build tree (relatively
     untested currently) (although pcc fixed this recently for old llvm-config).

llvm-svn: 143616