[WebAssembly] Provide WasmFunction content offset information.

WasmObjectWriter mostly operates with function segments offsets that do
not include their size fields. WasmObjectFile needs to have and provide
this information to the lld to maintain proper
R_WEBASSEMBLY_FUNCTION_OFFSET_I32 relocations entries.

Patch by Yury Delendik

Differential Revision: https://reviews.llvm.org/D46763

llvm-svn: 332406