[CMake][TableGen] Simplify code by using list(TRANSFORM)

[tambre]

Summary

LLVM requires CMake 3.13.4 so now we can simplify the code.

[phosek]

LGTM

llvm/cmake/modules/TableGen.cmake

if (CMAKE_GENERATOR MATCHES "Visual Studio")
     # Visual Studio has problems with llvm-tblgen's native --write-if-changed
     # behavior. Since it doesn't do restat optimizations anyway, just don't
     # pass --write-if-changed there.
     set(tblgen_change_flag)
   else()
     set(tblgen_change_flag "--write-if-changed")
   endif()
 
-  # With CMake 3.12 this can be reduced to:
-  # get_directory_property(tblgen_includes "INCLUDE_DIRECTORIES")
-  # list(TRANSFORM tblgen_includes PREPEND -I)
-  set(tblgen_includes)
-  get_directory_property(includes "INCLUDE_DIRECTORIES")
-  foreach(include ${includes})
-    list(APPEND tblgen_includes -I ${include})
-  endforeach()
   # We need both _TABLEGEN_TARGET and _TABLEGEN_EXE in the  DEPENDS list
   # (both the target and the file) to have .inc files rebuilt on
   # a tablegen change, as cmake does not propagate file-level dependencies
   # of custom targets. See the following ticket for more information:
   # https://cmake.org/Bug/view.php?id=15858
   # The dependency on both, the target and the file, produces the same
   # dependency twice in the result file when
   # ("${${project}_TABLEGEN_TARGET}" STREQUAL "${${project}_TABLEGEN_EXE}")
   # but lets us having smaller and cleaner code here.
+  get_directory_property(tblgen_includes INCLUDE_DIRECTORIES)
+  list(TRANSFORM tblgen_includes PREPEND -I)
+
   add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
     COMMAND ${${project}_TABLEGEN_EXE} ${ARGN} -I ${CMAKE_CURRENT_SOURCE_DIR}
     ${tblgen_includes}
     ${LLVM_TABLEGEN_FLAGS}
     ${LLVM_TARGET_DEFINITIONS_ABSOLUTE}
     ${tblgen_change_flag}
     ${additional_cmdline}
     # The file in LLVM_TARGET_DEFINITIONS may be not in the current