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

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.

Differential Revision: https://reviews.llvm.org/D119069

mlir/cmake/modules/AddMLIR.cmake

@@ -321,18 +321,11 @@ function(add_mlir_aggregate name)
       "    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}