[WebAssembly] MC: Remove SetUsed argument when calling MCSymbol::isDefined et al

Summary:
This argument (the isUsed flag) seems to only be relevant
when parsing.  Other calls sites such as these don't seem
to ever use it.

Subscribers: jfb, dschuff, jgravelle-google, aheejin, sunfish

Differential Revision: https://reviews.llvm.org/D41970

llvm-svn: 322332