[WebAssembly] Correctly handle va_arg of zero-sized structures

Summary:
D66168 passes size 0 structs indirectly, while the wasm backend expects it to
be passed directly. This causes subsequent variadic arguments to be read
incorrectly.

This diff changes it so that size 0 structs are passed directly.

Reviewers: dschuff, tlively, sbc100

Reviewed By: dschuff

Subscribers: jgravelle-google, aheejin, sunfish, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66255

llvm-svn: 369042