[WebAssembly] Do not use inheritance to define SubSection.

SubSection inherited from SyntheticSection, and SyntheticSection inherits
from OutputSection, so SubSection was an OutputSection. But that's wrong
because SubSection is not actually a WebAssembly output section.
It shares some functionalities with OutputSection, but overall it's very
different.

This patch removes that inheritance.

Differential Revision: https://reviews.llvm.org/D43719

llvm-svn: 326286