[WebAssembly] Add unimplemented-simd128 subtarget feature

Summary:
This replaces the old ad-hoc -wasm-enable-unimplemented-simd
flag. Also makes the new unimplemented-simd128 feature imply the
simd128 feature.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits, alexcrichton

Differential Revision: https://reviews.llvm.org/D56501

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350778 91177308-0d34-0410-b5e6-96231b3b80d8
21 files changed