TableGen: Allow foreach in multiclass to depend on template args
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
Reviewers: tra, simon_tatham, craig.topper, MartinO, arsenm
Subscribers: wdng, llvm-commits
Differential Revision: https://reviews.llvm.org/D47431