[WebAssembly] Add static_assert(sizeof(SymbolUnion) <= 96)

On Windows, the bitfield layout rule places `ussigned Referenced : 1` at
byte offset 40, instead of byte offset 37 on *NIX. The consequence is that
sizeof(SymbolUnion) == 104 on Windows while 96 on *NIX.

To eliminate this difference, change these unsigned bitfields to bool.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D64238

llvm-svn: 365296