Adopt SwiftABIInfo for WebAssembly.

Summary:
 - This adopts SwiftABIInfo as the base class for WebAssemblyABIInfo, which is in keeping with what is done for other targets for which Swift is supported.

 - This is a minimal patch to unblock exploration of WASM support for Swift (https://bugs.swift.org/browse/SR-9307)

Reviewers: rjmccall, sunfish

Reviewed By: rjmccall

Subscribers: ahti, dschuff, sbc100, jgravelle-google, aheejin, cfe-commits

Differential Revision: https://reviews.llvm.org/D56188

llvm-svn: 350372