[lld-macho][nfc] Add test for resolution of bitcode symbols

We lacked a test for bitcode symbol precedence. We assumed that
they followed the same rules as their regular symbol counterparts, but
never had a test to verify that we were matching ld64's behavior. It
turns out that we were largely correct, though we deviate from ld64 when
there are bitcode and non-bitcode symbols of the same name. The test
added in this diff both verifies our behavior and documents the
differences.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D106596