[lld][WebAssembly] Return 0 for synthetic function offsets (#96134)

When two or more functions' signatures differ, one of them is selected
and for other signatures `unreachable` stubs are generated:
https://github.com/llvm/llvm-project/blob/57778ec36c9c7e96b76a167f19dccbe00d49c9d4/lld/wasm/SymbolTable.cpp#L975
https://github.com/llvm/llvm-project/blob/57778ec36c9c7e96b76a167f19dccbe00d49c9d4/lld/wasm/SymbolTable.cpp#L852-L870

And when these `SyntheticFunction`s are generated, this constructor is
used,

https://github.com/llvm/llvm-project/blob/57778ec36c9c7e96b76a167f19dccbe00d49c9d4/lld/wasm/InputChunks.h#L266-L269
which does not set its `function` field:

https://github.com/llvm/llvm-project/blob/57778ec36c9c7e96b76a167f19dccbe00d49c9d4/lld/wasm/InputChunks.h#L304
As a result, the `function` field contains a garbage value for these
stub functions.

`InputFunction::getFunctionCodeOffset()` is called when relocations are
resolved for `.debug_info` section to get functions' PC locations. But
because these stub functions don't have their `function` field set, this
function segfaults:

https://github.com/llvm/llvm-project/blob/57778ec36c9c7e96b76a167f19dccbe00d49c9d4/lld/wasm/InputChunks.h#L282

This bug seems to be triggered when these conditions are met:
- There is a signature mismatch warning with multiple different
definitions (one definition with other declarations is not sufficient)
with weak linkage with the same name
- The 'stub' function containing unreachable has a callsite, meaning it
isn't DCE'd
- .debug_info section is generated (i.e., DWARF is used)

This PR initializes the field with `nullptr`, and in
`InputFunction::getFunctionCodeOffset`, checks if `function` is
`nullptr`, and if so, just returns 0. This function is called only for
resolving relocations in the `.debug_info` section, and addresses of
these stub functions, which are not the functions users wrote in the
first place, are not really meaningful anyway.