[mlir] Allow CXX standard to be overridden (#112957)

MLIR previously hardcoded the CXX version to C++17. Updated to allow for
the CXX version to be set by clients (mirrors other LLVM projects).

Author: nikalra

mlir/CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
 
 if(MLIR_STANDALONE_BUILD)
   find_package(LLVM CONFIG REQUIRED)