[WebAssembly] Added visibility and ident directives to WasmAsmParser.

Summary:
These are output by clang -S, so can now be roundtripped thru clang.

(partially) fixes: https://bugs.llvm.org/show_bug.cgi?id=34544

Reviewers: dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63901

llvm-svn: 364658