[WebAssembly] Add unimplemented-simd128 subtarget feature

Summary:
This is a third attempt, but this time we have vetted it on Windows
first. The previous errors were due to an uninitialized class member.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D56560

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350901 91177308-0d34-0410-b5e6-96231b3b80d8
23 files changed