[TableGen] Don't quote variable name when printing !foreach.

An input !foreach expression such as !foreach(a, lst, !add(a, 1))
would be re-emitted by llvm-tblgen -print-records with the first
argument in quotes, giving !foreach("a", lst, !add(a, 1)), which isn't
valid TableGen input syntax.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46352

llvm-svn: 331351