[lld] Make error handling functions opaque

The inline `lld::error` expands to two function calls `errorHandler` and `error`
where the latter is opaque. Move the functions to .cpp files to decrease code
size.

My x86-64 lld executable is 9KiB smaller.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D120002