[cmake] Place clang behind mlir in the liist of external projects

In preparation for the initial ClangIR upstreaming process, move clang
behind MLIR in the list of external projects. Otherwise, cmake will
attempt to build clang before MLIR.

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

Reviewers: 

Pull Request: https://github.com/llvm/llvm-project/pull/86210