llvm-config: Let directories aware of CMAKE_CFG_INTDIR.

With llvm-config.exe --bindir --libdir --build-mode, on Visual Studio 2010,

In build tree:
  (OBJ_ROOT)/bin/MinSizeRel
  (OBJ_ROOT)/lib/MinSizeRel
  MinSizeRel

In installed tree:
  (INSTALL_PREFIX)/bin
  (INSTALL_PREFIX)/lib
  MinSizeRel

This is enhancements since r196283.