[CMake] Propagate dependencies to OBJECT libraries in add_llvm_library (re-land) (#184201)

Previously, transitively inherited calls to
`target_include_directories(foo SYSTEM ...)` were being squashed into a
flat list of includes, effectively stripping off `-isystem` and
unintentionally forwarding warnings from such dependencies.

To correctly propagate `SYSTEM` dependencies, use
`target_link_libraries` to forward the parent target's link dependencies
to the OBJECT library (similar to the `_static` flow below). Unlike a
flat `target_include_directories`, this lets CMake resolve transitive
SYSTEM include directories through the proper dependency chain.

Note that `target_link_libraries` on an OBJECT library propagates all
usage requirements, not just includes. This also brings in transitive
`INTERFACE_COMPILE_DEFINITIONS`, `INTERFACE_COMPILE_OPTIONS`, and
`INTERFACE_COMPILE_FEATURES`. This is arguably more correct, as the
OBJECT library compiles the same sources and should see the same flags.

The existing `target_include_directories` call is retained for include
directories set directly on the target (not through link dependencies).
CMake deduplicates include directories that appear through both paths.
Compile definitions and options may technically appear twice (once via
the OBJECT library, once via the consuming target), but duplicate `-D`
and flag entries are harmless in practice.

Also fix `clang_target_link_libraries` and `mlir_target_link_libraries`
to forward the link type (PUBLIC/PRIVATE/INTERFACE) to `obj.*` targets.
Previously the type keyword was silently dropped, resulting in plain-
signature `target_link_libraries` calls. This is now required because
the new keyword-signature call in `llvm_add_library` would otherwise
conflict (CMake requires all calls on a target to use the same
signature).

clang/cmake/modules/AddClang.cmake

@@ -217,7 +217,7 @@ endmacro()
 
 function(clang_target_link_libraries target type)
   if (TARGET obj.${target})
-    target_link_libraries(obj.${target} ${ARGN})
+    target_link_libraries(obj.${target} ${type} ${ARGN})
   endif()
 
   get_property(LLVM_DRIVER_TOOLS GLOBAL PROPERTY LLVM_DRIVER_TOOLS)

lld/tools/lld/CMakeLists.txt

@@ -13,7 +13,7 @@ export_executable_symbols_for_plugins(lld)
 
 function(lld_target_link_libraries target type)
   if (TARGET obj.${target})
-    target_link_libraries(obj.${target} ${ARGN})
+    target_link_libraries(obj.${target} ${type} ${ARGN})
   endif()
 
   get_property(LLVM_DRIVER_TOOLS GLOBAL PROPERTY LLVM_DRIVER_TOOLS)

llvm/cmake/modules/AddLLVM.cmake

@@ -636,7 +636,12 @@ function(llvm_add_library name)
     # Do add_dependencies(obj) later due to CMake issue 14747.
     list(APPEND objlibs ${obj_name})
 
-    # Bring in the target include directories from our original target.
+    # Bring in the target include directories and link info from our original
+    # target. target_link_libraries propagates transitive dependencies with
+    # proper SYSTEM include handling from IMPORTED targets.
+    # target_include_directories propagates include directories set directly on
+    # the target.
+    target_link_libraries(${obj_name} PRIVATE $<TARGET_PROPERTY:${name},LINK_LIBRARIES>)
     target_include_directories(${obj_name} PRIVATE $<TARGET_PROPERTY:${name},INCLUDE_DIRECTORIES>)
 
     set_target_properties(${obj_name} PROPERTIES FOLDER "${subproject_title}/Object Libraries")
@@ -1117,7 +1122,7 @@ macro(generate_llvm_objects name)
 
       set_property(GLOBAL APPEND PROPERTY LLVM_DRIVER_TOOLS ${name})
       set_property(GLOBAL APPEND PROPERTY LLVM_DRIVER_TOOL_ALIASES_${name} ${name})
-      target_link_libraries(${obj_name} ${LLVM_PTHREAD_LIB})
+      target_link_libraries(${obj_name} PUBLIC ${LLVM_PTHREAD_LIB})
       llvm_config(${obj_name} ${USE_SHARED} ${LLVM_LINK_COMPONENTS} )
     endif()
   endif()

llvm/cmake/modules/LLVM-Config.cmake

@@ -106,7 +106,10 @@ function(explicit_llvm_config executable)
   get_target_property(t ${executable} TYPE)
   if(t STREQUAL "STATIC_LIBRARY")
     target_link_libraries(${executable} INTERFACE ${LIBRARIES})
-  elseif(t STREQUAL "EXECUTABLE" OR t STREQUAL "SHARED_LIBRARY" OR t STREQUAL "MODULE_LIBRARY")
+  elseif(t STREQUAL "EXECUTABLE" OR
+         t STREQUAL "SHARED_LIBRARY" OR
+         t STREQUAL "MODULE_LIBRARY" OR
+         t STREQUAL "OBJECT_LIBRARY")
     target_link_libraries(${executable} PRIVATE ${LIBRARIES})
   else()
     # Use plain form for legacy user.

mlir/cmake/modules/AddMLIR.cmake

@@ -769,7 +769,7 @@ endfunction(mlir_check_all_link_libraries)
 # used.
 function(mlir_target_link_libraries target type)
   if (TARGET obj.${target})
-    target_link_libraries(obj.${target} ${ARGN})
+    target_link_libraries(obj.${target} ${type} ${ARGN})
   endif()
 
   if (MLIR_LINK_MLIR_DYLIB)