swift234

Make sure that even in the standalone build configuration, we can build doxygen documentation.

The issue is that this was originally done when in-tree builds were the
preferred way of building so LLVM_ENABLE_DOXYGEN would be defined and LLVM would
have found doxygen as well. When one is doing the current preferred standalone
install, the option LLVM_ENABLE_DOXYGEN is not discoverable to the user via an
option with a default argument and the doxygen package is never searched for.

This commit ensures that when building standalone:

1. LLVM_ENABLE_DOXYGEN is defined as an option with a default value of FALSE.
2. The cmake dtrace package is searched for and found.

When we are building standalone, we take these values from LLVM.