[demangler] Fix unresolvedname demangling

We were dropping the [gs] modifier by parsing it in parseExpr, but not
forwarding it on to parseUnresolvedName.  This is the straightforwards
fix to forward that flag -- parseExpr must see past it.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D118504

GitOrigin-RevId: 8d38273a3dae41e8da381542a20fbd09d4ccedc2