Drop the mig-require-libz3 patch since upstream removed that test. Two new
tests need handling for our Z3-less external-llvm build: disable Z3
discovery with Z3_DISABLE so the z3 integration tests skip instead of
running against a circt that has no Z3 backend, and symlink circt's empty
llvm submodule at the real tree so the circt-tblgen tests find their mlir
tablegen includes.
Signed-off-by: Huang Rui <vowstar@gmail.com>
