Add a CMake fatal error when someone tries to configure flang without MLIR

[mehdi_amini]

Summary

This is more friendly than the "Unknown CMake command “mlir_tablegen”."
that would be issued instead.

[stephenneuendorffer]

Maybe just add mlir to the list of enabled projects automatically?

[mehdi_amini]

Add the dependency automatically instead

[DavidTruby]

LGTM

llvm/CMakeLists.txt

 set(LLVM_EXTRA_PROJECTS "flang")
 # List of all known projects in the mono repo
 set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
   "Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()
+if ("flang" IN_LIST LLVM_ENABLE_PROJECTS AND NOT "mlir" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(STATUS "Enabling MLIR as a dependency to flang")
+  list(APPEND LLVM_ENABLE_PROJECTS "mlir")
+endif()
 
 # LLVM_ENABLE_PROJECTS_USED is `ON` if the user has ever used the
 # `LLVM_ENABLE_PROJECTS` CMake cache variable.  This exists for
 # several reasons:
 #
 # * As an indicator that the `LLVM_ENABLE_PROJECTS` list is now the single
 # source of truth for which projects to build. This means we will ignore user
 # supplied `LLVM_TOOL_<project>_BUILD` CMake cache variables and overwrite