[cmake] Follow LLVM's lead in creating exported tool targets for clang tools.

This is needed by downstream projects such as swift to get proper cmake
dependency information for LLVM/Clang targets.

A few months ago I added support for exporting this information for Clang
libraries. In order to be incremental, I did not add support for exporting clang
tools as well at that time. Now such support is needed, so I am committing this
incremental code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@284658 91177308-0d34-0410-b5e6-96231b3b80d8