[mlir][cmake] Fix missing entries in tablegen_compile_commands.yml (#147516)

Depending on the order of CMake includes the
`tablegen_compile_commands.yml` was previously missing entries due to
being deleted after a `tablegen` commands.

This PR fixes this by 1) making sure `AddMLIR` includes `TableGen` such
that new compile commands are added to a fresh YML file and 2) using an
include guard to ensure the file is cleared exactly once

Author: zero9178

llvm/cmake/modules/TableGen.cmake

@@ -4,10 +4,6 @@
 # Adds the name of the generated file to TABLEGEN_OUTPUT.
 include(LLVMDistributionSupport)
 
-# Clear out any pre-existing compile_commands file before processing. This
-# allows for generating a clean compile_commands on each configure.
-file(REMOVE ${CMAKE_BINARY_DIR}/tablegen_compile_commands.yml)
-
 function(tablegen project ofn)
   cmake_parse_arguments(ARG "" "" "DEPENDS;EXTRA_INCLUDES" ${ARGN})
 
@@ -250,3 +246,11 @@ macro(add_tablegen target project)
     set_property(GLOBAL APPEND PROPERTY ${export_upper}_EXPORTS ${target})
   endif()
 endmacro()
+
+# Make sure 'tablegen_compile_commands.yml' is only deleted once the very
+# first time this file is included.
+include_guard(GLOBAL)
+
+# Clear out any pre-existing compile_commands file before processing. This
+# allows for generating a clean compile_commands on each configure.
+file(REMOVE ${CMAKE_BINARY_DIR}/tablegen_compile_commands.yml)

mlir/cmake/modules/AddMLIR.cmake

@@ -1,3 +1,4 @@
+include(TableGen)
 include(GNUInstallDirs)
 include(LLVMDistributionSupport)