TableGen: Allow foreach in multiclass to depend on template args

Summary:
This also allows inner foreach loops to have a list that depends on
the iteration variable of an outer foreach loop. The test cases show
some very simple examples of how this can be used.

This was perhaps the last remaining major non-orthogonality in the
TableGen frontend.

Change-Id: I79b92d41a5c0e7c03cc8af4000c5e1bda5ef464d

Reviewers: tra, simon_tatham, craig.topper, MartinO, arsenm

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D47431

llvm-svn: 335221