[MLIR] Move from add_dependencies() to DEPENDS

add_llvm_library and add_llvm_executable may need to create new targets with
appropriate dependencies.  As a result, it is not sufficient in some
configurations (namely LLVM_BUILD_LLVM_DYLIB=on) to only call
add_dependencies().  Instead, the explicit TableGen dependencies must
be passed to add_llvm_library() or add_llvm_executable() using the DEPENDS
keyword.

Differential Revision: https://reviews.llvm.org/D74930