[WebAssembly] Ensure BasicSymbolRef.getRawDataRefImpl().p is non-null

Store a non-zero value to ref.d.a and use ref.d.b to store the symbol
index.  This means that ref.p is never null, which was confusing
llvm-nm.

Fixes PR40497

Differential Revision: https://reviews.llvm.org/D57373

llvm-svn: 352551