The patch failed to apply on 18 and 19. We have to version it.
LLVM 18 has no MLIR_SRC_SHARDER_TABLEGEN_EXE. LLVM 19 and 20 fail to
apply the 21+ patch due to mechanical hunk failures, but otherwise use
an identical patch to 21+.
MLIRConfig.cmake unconditionally overwrites MLIR_TABLEGEN_EXE and does
not create an imported target, breaking standalone builds that provide
their own mlir-tblgen binary (e.g. Nix sandboxed builds).
The patch adds guards to respect caller-set MLIR_TABLEGEN_EXE,
MLIR_PDLL_TABLEGEN_EXE, and MLIR_SRC_SHARDER_TABLEGEN_EXE values, and
auto-creates an imported mlir-tblgen target for downstream consumers.
This replaces the previous dummy-target workaround in flang's
CMakeLists.txt.
Upstream issue: https://github.com/llvm/llvm-project/issues/150986
Co-authored-by: Alexander Bantyev <balsoft@balsoft.ru>
