[WebAssembly] Parse llvm.ident into producers section

Summary:
Everything before the word "version" is the tool, and everything after
the word "version" is the version.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D56742

llvm-svn: 351399