CMake: Automatic regeneration of the library dependencies file.

It doesn't stop or reconfigure the build, though, so the user will see
a broken build that magically succeeds at the next attempt. It is
technically possible to halt the build with a helpful message, and
even to automatically restart the build using the new dependencies as
it we did when llvm-config was used by cmake for learning
dependencies. This is left on the TODO list.

llvm-svn: 79004