Introduce a new loader method "SetCarrierSym", to be used when
establishing container/containee symbol relationships for symbol
bucketing in the symtab phase.

This new method is intended to be employed in situations where you
have a series of related symbols will be represented by a single
carrier symbol as a combined entity. The pattern here is that the
sub-symbols contain content but will be anonymous from a symbol table
perspective; the carrier symbol has no content itself but will appear
in the symbol table. Examples of carrier symbols that follow this
model are "runtime.itablink" and "runtime.typelink".

Change-Id: I1a3391a71062c7c740cb108b3fa210b7f69b81ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/240509


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>