[libc] Improve get_object_files_for_test to reduce CMake configure time for tests. (#75552)

Profiling cmake shows that a significant time configuring `libc` folder
is spent on running `get_object_files_for_test` in the `test` folder (13
sec in `libc/test` folder / 16 sec in `libc` folder). By caching all
needed objects for each target instead of resolving every time, the time
cmake spends on configuring `libc/test` folder is reduced to ~1s.

Author: lntue

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -18,62 +18,81 @@ function(get_object_files_for_test result skipped_entrypoints_list)
   set(checked_list "")
   set(unchecked_list "${ARGN}")
   list(REMOVE_DUPLICATES unchecked_list)
-  list(LENGTH unchecked_list length)
 
-  while(length)
-    set(indirect_list "")
+  foreach(dep IN LISTS unchecked_list)
+    if (NOT TARGET ${dep})
+      # Skip tests with undefined dependencies.
+      list(APPEND skipped_list ${dep})
+      continue()
+    endif()
+    get_target_property(aliased_target ${dep} "ALIASED_TARGET")
+    if(aliased_target)
+      # If the target is just an alias, switch to the real target.
+      set(dep ${aliased_target})
+    endif()
 
-    foreach(dep IN LISTS unchecked_list)
-      if (NOT TARGET ${dep})
-        # Skip tests with undefined dependencies.
-        list(APPEND skipped_list ${dep})
-        continue()
-      endif()
-      get_target_property(dep_type ${dep} "TARGET_TYPE")
-      if(NOT dep_type)
-        # Skip tests with no object dependencies.
-        continue()
-      endif()
+    get_target_property(dep_type ${dep} "TARGET_TYPE")
+    if(NOT dep_type)
+      # Skip tests with no object dependencies.
+      continue()
+    endif()
+
+    get_target_property(dep_checked ${dep} "CHECK_OBJ_FOR_TESTS")
+
+    if(dep_checked)
+      # Target full dependency has already been checked.  Just use the results.
+      get_target_property(dep_obj ${dep} "OBJECT_FILES_FOR_TESTS")
+      get_target_property(dep_skip ${dep} "SKIPPED_LIST_FOR_TESTS")
+    else()
+      # Target full dependency hasn't been checked.  Recursively check its DEPS.
+      set(dep_obj "${dep}")
+      set(dep_skip "")
+
+      get_target_property(indirect_deps ${dep} "DEPS")
+      get_object_files_for_test(dep_obj dep_skip ${indirect_deps})
 
       if(${dep_type} STREQUAL ${OBJECT_LIBRARY_TARGET_TYPE})
         get_target_property(dep_object_files ${dep} "OBJECT_FILES")
         if(dep_object_files)
-          list(APPEND object_files ${dep_object_files})
+          list(APPEND dep_obj ${dep_object_files})
         endif()
       elseif(${dep_type} STREQUAL ${ENTRYPOINT_OBJ_TARGET_TYPE})
         get_target_property(is_skipped ${dep} "SKIPPED")
         if(is_skipped)
-          list(APPEND skipped_list ${dep})
-          continue()
+          list(APPEND dep_skip ${dep})
+          list(REMOVE_ITEM dep_obj ${dep})
         endif()
         get_target_property(object_file_raw ${dep} "OBJECT_FILE_RAW")
         if(object_file_raw)
-          list(APPEND object_files ${object_file_raw})
+          list(APPEND dep_obj ${object_file_raw})
         endif()
       elseif(${dep_type} STREQUAL ${ENTRYPOINT_OBJ_VENDOR_TARGET_TYPE})
         # Skip tests for externally implemented entrypoints.
-        list(APPEND skipped_list ${dep})
-        continue()
+        list(APPEND dep_skip ${dep})
+        list(REMOVE_ITEM dep_obj ${dep})
       endif()
 
-      get_target_property(indirect_deps ${dep} "DEPS")
-      list(APPEND indirect_list "${indirect_deps}")
-    endforeach(dep)
+      set_target_properties(${dep} PROPERTIES
+        OBJECT_FILES_FOR_TESTS "${dep_obj}"
+        SKIPPED_LIST_FOR_TESTS "${dep_skip}"
+        CHECK_OBJ_FOR_TESTS "YES"
+      )
+
+    endif()
+
+    list(APPEND object_files ${dep_obj})
+    list(APPEND skipped_list ${dep_skip})
 
-    # Only add new indirect dependencies to check.
-    list(APPEND checked_list "${unchecked_list}")
-    list(REMOVE_DUPLICATES indirect_list)
-    list(REMOVE_ITEM indirect_list checked_list)
-    set(unchecked_list "${indirect_list}")
-    list(LENGTH unchecked_list length)
-  endwhile()
+  endforeach(dep)
 
   list(REMOVE_DUPLICATES object_files)
   set(${result} ${object_files} PARENT_SCOPE)
   list(REMOVE_DUPLICATES skipped_list)
   set(${skipped_entrypoints_list} ${skipped_list} PARENT_SCOPE)
+
 endfunction(get_object_files_for_test)
 
+
 # Rule to add a libc unittest.
 # Usage
 #    add_libc_unittest(