[cmake] Created an aggregate doxygen target for generating doxygen documentation for llvm/all subprojects. Renamed llvm's doxygen generation command to doxygen-llvm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189506 91177308-0d34-0410-b5e6-96231b3b80d8
[doxygen] Added support for doxygen external search.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189507 91177308-0d34-0410-b5e6-96231b3b80d8