[WebAssembly] Check if the section order is correct

Summary:
This patch checks if the section order is correct when reading a wasm
object file in `WasmObjectFile` and converting YAML to wasm object in
yaml2wasm. (It is not possible to check when reading YAML because it is
handled exclusively by the YAML reader.)

This checks the ordering of all known sections (core sections + known
custom sections). This also adds section ID DataCount section that will
be scheduled to be added in near future.

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D54924

llvm-svn: 349221