[MLIR] Make MLIR_TABLEGEN_EXE a CMake CACHE variable

[vinograd47]

Summary

This allows to explicitly override this variable during cross-compilation to use native mlir-tblgen tool.

[mehdi_amini]

Is this fixed with https://reviews.llvm.org/D75299 ?

[vinograd47]

In D75228#1897299, @mehdi_amini wrote:

Is this fixed with https://reviews.llvm.org/D75299 ?

I think so, since my fix also relates to cross-compilation. Once the D75299 is merged I will try it.

mlir/CMakeLists.txt

 # MLIR project.
 set(MLIR_MAIN_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include ) # --src-root
 set(MLIR_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/include ) # --includedir
-set(MLIR_TABLEGEN_EXE mlir-tblgen)
+set(MLIR_TABLEGEN_EXE mlir-tblgen CACHE STRING "")
 
 set(MLIR_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(MLIR_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 function(add_mlir_dialect dialect dialect_doc_filename)
   set(LLVM_TARGET_DEFINITIONS ${dialect}.td)