Move llvm Options.inc hack from r321433 for NO_CLEAN to lib/clang/libllvm.

The files are only ever generated to .OBJDIR, not to WORLDTMP (as a
sysroot) and are only ever included from a compilation. So using
a beforebuild target here removes the file before the compilation
tries to include it.

MFC after: 2 months
X-MFC-With: r321369