Reland "[lld][WebAssembly] Cleanup duplicate fields in Symbols.h. NFC"

This avoids duplication and simplifies the code in several places
without increasing the size of the symbol union (at least not
above the assert'd limit of 120 bytes).

Originally commit: 9b965b37
Reverted in: 16aac493.

Differential Revision: https://reviews.llvm.org/D106026