[mlir] Prepend include directories before LLVM includes (llvm#121223)

Prepend mlir's include directories before system LLVM include
directories. This is particularly important for standalone builds, where
system include directory may contain the previous version of mlir, and
therefore various mlir targets (particularly tablegen) end up using the
headers from the previous version over the fresh ones. The new logic is
copied from clang.

mlir/CMakeLists.txt

@@ -196,8 +196,10 @@ endif()
 
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
-include_directories( "include")
-include_directories( ${MLIR_INCLUDE_DIR})
+include_directories(BEFORE
+  "include"
+  ${MLIR_INCLUDE_DIR}
+  )
 
 # Adding tools/mlir-tblgen here as calling add_tablegen sets some variables like
 # MLIR_TABLEGEN_EXE in PARENT_SCOPE which gets lost if that folder is included