[WebAssembly] Add COMDAT support

This adds COMDAT support to the Wasm object-file format.
Spec: https://github.com/WebAssembly/tool-conventions/pull/31

Corresponding LLD change:
https://bugs.llvm.org/show_bug.cgi?id=35533, and D40845

Patch by Nicholas Wilson

Differential Revision: https://reviews.llvm.org/D40844

llvm-svn: 322135