[CMake] Use target_link_libraries(INTERFACE|PRIVATE) on CMake-2.8.12 to increase opportunity for parallel build.

target_link_libraries(INTERFACE) doesn't bring inter-target dependencies in add_library,
although final targets have dependencies to whole dependent libraries.
It makes most libraries can be built in parallel.

target_link_libraries(PRIVATE) is used to shaared library.
Each dependent library is linked to the target.so, and its user will not see its grandchildren.
For example,

  - libclang.so has sufficient libclang*.a(s).
  - c-index-test requires just only libclang.so.

FIXME: lld is tweaked minimally. Adding INTERFACE in each library would be better thing.
llvm-svn: 202241