Revert "[OpenCL] Move OpenCLBuiltins.td and remove unused include"

This reverts commit r363242 as it broke some builds with

  make[2]: *** No rule to make target 'ClangOpenCLBuiltinsImpl', needed by
  'tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaLookup.cpp.o'.

llvm-svn: 363376