[doc]: build ocaml_doc by default to avoid missing install artifacts

Mark `ocaml_doc` as an ALL target so that the HTML docs are generated
before `cmake --build . --target install`. This prevents `install(DIRECTORY …)`
from failing when the ocamldoc output is absent.

Author: Acture

llvm/docs/CMakeLists.txt

@@ -136,7 +136,7 @@ if( NOT uses_ocaml LESS 0 AND LLVM_ENABLE_OCAMLDOC )
     list(APPEND odoc_files -load ${odoc_file})
   endforeach()
 
-  add_custom_target(ocaml_doc
+  add_custom_target(ocaml_doc ALL
     COMMAND ${CMAKE_COMMAND} -E remove_directory ${CMAKE_CURRENT_BINARY_DIR}/ocamldoc/html
     COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/ocamldoc/html
     COMMAND ${OCAMLFIND} ocamldoc -d ${CMAKE_CURRENT_BINARY_DIR}/ocamldoc/html