Add set(CMAKE_CXX_STANDARD 17) to MLIR CMakeLists.txt

This is only useful when building the project in a "standalone" way: that is by
invoking cmake pointing at mlir/ to build against an already built LLVM.

Fixes #60574

Author: joker-eph

mlir/CMakeLists.txt

@@ -22,6 +22,7 @@ endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
+set(CMAKE_CXX_STANDARD 17)
 
 if(MLIR_STANDALONE_BUILD)
   find_package(LLVM CONFIG REQUIRED)