[clang] Fix clean build of generate-order-file

This diff fixes the clean build of the target generate-order-file.
In llvm/tools/clang/CMakeLists.txt 
add_subdirectory(utils/perf-training) should go after the block where 
the value of the variable CLANG_ORDER_FILE is set - otherwise 
(tested with cmake's version 3.6.2) the arguments of perf-helper.py gen-order-file
will be ill-formed (CLANG_ORDER_FILE will be empty).

Differential revision: https://reviews.llvm.org/D28153

llvm-svn: 290781