tblgen: add preprocessor as a separate mode

This patch adds a preprocessor that can expand nested for-loops for
saving some copy-n-paste in *.td files.

The preprocessor is not yet integrated with TGParser, and so it has
no direct effect on *.td inputs.  However, you may preprocess an td
input (and only preprocess it).

To test the proprecessor, type:
  tblgen -E -o $@ $<

llvm-svn: 141079