[Symbolizer][WebAssembly] Use wasm-specific getSymbolSize (#82083)

getSymbolSize was recently added to WasmObjectFile and has correct sizes 
for most symbol types. This makes llvm-symbolizer correctly symbolize
addresses in the middle of the symbol.

When reworking the test I also noticed that the DWARF info seems to be
wrong for the first instruction in each function. I noted that in the test
comments but didn't attempt to fix here.