TableGen: Allow implicit casting between string and code

Summary:
Perhaps the distinction between the two should be removed entirely
in the long term, and the [{ ... }] syntax should just be a convenient
way of writing multi-line strings.

In the meantime, a lot of existing .td files are quite relaxed about
string vs. code, and this change allows switching on more consistent
type checks without breaking those.

Change-Id: If85e3e04469e41b58e2703b62ac0032d2711713c

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43557

llvm-svn: 325799