[runtimes] Generalize how we reorder projects

This way, we could use it for LLVM_ENABLE_PROJECTS too if desired.

Differential Revision: https://reviews.llvm.org/D125121

cmake/Modules/SortSubset.cmake

@@ -0,0 +1,25 @@
+# Sort a subset of a list according to the ordering in the full list.
+#
+# Given a list and a subset of that list, this function sorts the subset
+# according to the order in the full list, and returns that in the given
+# output variable.
+#
+# full_list:
+#   The list containing the desired order of elements in the sub-list.
+#
+# sub_list:
+#   A subset of the elements in `full_list`. Those elements will be sorted
+#   according to the order in `full_list`.
+#
+# out_var:
+#   A variable to store the resulting sorted sub-list in.
+function(sort_subset full_list sub_list out_var)
+  set(result "${full_list}")
+  foreach(project IN LISTS full_list)
+    if (NOT project IN_LIST sub_list)
+      list(REMOVE_ITEM result ${project})
+    endif()
+  endforeach()
+
+  set(${out_var} "${result}" PARENT_SCOPE)
+endfunction()