[WebAssembly] Change event section code to 13

Summary:
We decided to change the event section code from 12 to 13 as new
`DataCount` section in the bulk memory operations proposal will take the
code 12 instead.

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D55343

llvm-svn: 348424