[lld-macho] Add support for N_INDR symbols

This is similar to the `-alias` CLI option, but it gives finer-grained
control in that it allows the aliased symbols to be treated as private
externs.

While working on this, I realized that our `-alias` handling did not
cover the cases where the aliased symbol is a common or dylib symbol,
nor the case where we have an undefined that gets treated specially and
converted to a defined later on. My N_INDR handling neglects this too
for now; I've added checks and TODO messages for these.

`N_INDR` symbols cropped up as part of our attempt to link swift-stdlib.

Reviewed By: #lld-macho, thakis, thevinster

Differential Revision: https://reviews.llvm.org/D133825

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