[lld-macho] Partial support for weak definitions

This diff adds support for weak definitions, though it doesn't handle weak
symbols in dylibs quite correctly -- we need to emit binding opcodes for them
in the weak binding section rather than the lazy binding section.

What *is* covered in this diff:

1. Reading the weak flag from symbol table / export trie, and writing it to the
   export trie
2. Refining the symbol table's rules for choosing one symbol definition over
   another. Wrote a few dozen test cases to make sure we were matching ld64's
   behavior.

We can now link basic C++ programs.

Reviewed By: #lld-macho, compnerd

Differential Revision: https://reviews.llvm.org/D83532