[llvm] [cmake] Place clang behind mlir in the list of external projects (PR #86050)

[Omair Javaid via llvm-commits]

omjavaid wrote:

This breaks LLDB buildbots:
https://lab.llvm.org/buildbot/#/builders/96/builds/54822
https://lab.llvm.org/buildbot/#/builders/17/builds/50792
https://lab.llvm.org/buildbot/#/builders/219/builds/10031


https://github.com/llvm/llvm-project/pull/86050