[lld/mac] Don't assert during thunk insertion if there are undefined symbols

We end up calling resolveBranchVA(), which asserts for Undefineds.

As fix, just return early in Writer::run() if there are any diagnostics
after processing relocations (which is where undefined symbol errors are
emitted). This matches what the ELF port does.

Differential Revision: https://reviews.llvm.org/D109079