[WebAssembly] Simplify initializeSymbols and merge it with ObjFile::parse. NFC.

This patch simplifies initializeSymbols. Since that function is called
at the tail context of ObjFile::parse, and the function is called only
once from that function, that's effectively just a continuation of
ObjFile::parse. So this patch merge it with ObjFile::parse.

Differential Revision: https://reviews.llvm.org/D43848

llvm-svn: 326296