[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