[mlir] Do not use an empty source file when building aggregate libraries.

[stellaraccident]

Summary

See discussion: https://discourse.llvm.org/t/check-mlir-times-examples-standalone-testing-time/6073/7

It turns out that it has been legal since CMake 3.11 to omit sources at library creation time if they are added later via target_sources, as is done here. This side-steps the issue of having a file that changes and invalidates the build of leaves.

[mehdi_amini]

Thanks Stella!

mlir/cmake/modules/AddMLIR.cmake

foreach(lib ${ARG_EMBED_LIBS})
     list(APPEND _deps ${_local_deps})
 
     string(APPEND _debugmsg
       ": EMBED_LIB ${lib}:\n"
       "    OBJECTS = ${_local_objects}\n"
       "    DEPS = ${_local_deps}\n\n")
   endforeach()
 
-  # Unfortunately need to compile at least one source file, which is hard
-  # to guarantee, so just always generate one. We generate one vs using the
-  # LLVM common dummy.cpp because it works better out of tree.
-  set(_empty_src "${CMAKE_CURRENT_BINARY_DIR}/${name}__empty.cpp")
-  file(WRITE "${_empty_src}" "typedef int dummy;")
-
   add_mlir_library(${name}
     ${_libtype}
     ${ARG_UNPARSED_ARGUMENTS}
     PARTIAL_SOURCES_INTENDED
     EXCLUDE_FROM_LIBMLIR
-    "${_empty_src}"
     LINK_LIBS PRIVATE
     ${_deps}
     ${ARG_PUBLIC_LIBS}
   )
   target_sources(${name} PRIVATE ${_objects})
   # TODO: Should be transitive.
   set_target_properties(${name} PROPERTIES
     MLIR_AGGREGATE_EXCLUDE_LIBS "${_embed_libs}")