[llvm] [CMake] Add dependent project mode for Flang's MLIR (PR #221724)

[Renato Golin via llvm-commits]

================
@@ -187,6 +191,10 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
   if (NOT "mlir" IN_LIST LLVM_ENABLE_PROJECTS)
     message(STATUS "Enabling MLIR as a dependency to flang")
     list(APPEND LLVM_ENABLE_PROJECTS "mlir")
+    list(APPEND LLVM_DEPENDENT_PROJECTS "mlir")
----------------
rengolin wrote:

so `LLVM_DEPENDENT_PROJECTS` is _in addition_ to `LLVM_ENABLE_PROJECTS`? Just to mark it is a dep?

nit: perhaps call it `LLVM_DEPENDENCY_ONLY_PROJECTS`? So that it's clear that it needs the `ENABLE` to also be included?

https://github.com/llvm/llvm-project/pull/221724