Add missing mlir-headers target and add tablegen'd deps to it.

[stellaraccident]

Summary

Prior to this, "ninja install-mlir-headers" failed with an error indicating
the missing target. Verified that from a clean build, the installed
headers include generated files.

[jpienaar]

Thanks!

mlir/CMakeLists.txt

set(TABLEGEN_OUTPUT ${TABLEGEN_OUTPUT} ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
       PARENT_SCOPE)
 endfunction()
 
 function(add_mlir_dialect dialect dialect_doc_filename)
   set(LLVM_TARGET_DEFINITIONS ${dialect}.td)
   mlir_tablegen(${dialect}.h.inc -gen-op-decls)
   mlir_tablegen(${dialect}.cpp.inc -gen-op-defs)
   add_public_tablegen_target(MLIR${dialect}IncGen)
+  add_dependencies(mlir-headers MLIR${dialect}IncGen)
 
   # Generate Dialect Documentation
   set(LLVM_TARGET_DEFINITIONS ${dialect_doc_filename}.td)
   tablegen(MLIR ${dialect_doc_filename}.md -gen-op-doc "-I${MLIR_MAIN_SRC_DIR}" "-I${MLIR_INCLUDE_DIR}")
   set(GEN_DOC_FILE ${MLIR_BINARY_DIR}/docs/Dialects/${dialect_doc_filename}.md)
   add_custom_command(
           OUTPUT ${GEN_DOC_FILE}
           COMMAND ${CMAKE_COMMAND} -E copy
                   ${CMAKE_CURRENT_BINARY_DIR}/${dialect_doc_filename}.md
                   ${GEN_DOC_FILE}
           DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${dialect_doc_filename}.md)
   add_custom_target(${dialect_doc_filename}DocGen DEPENDS ${GEN_DOC_FILE})
   add_dependencies(mlir-doc ${dialect_doc_filename}DocGen)
 endfunction()
 
+# Installing the headers and docs needs to depend on generating any public
+# tablegen'd targets.
+add_custom_target(mlir-headers)
+set_target_properties(mlir-headers PROPERTIES FOLDER "Misc")
 add_custom_target(mlir-doc)
 
 # TODO: This is to handle the current static registration, but should be
 # factored out a bit.
 function(whole_archive_link target)
   if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
     set(link_flags "-L${CMAKE_BINARY_DIR}/lib ")
     FOREACH(LIB ${ARGN})