For ThinLTO we only have the cloning information in the FunctionSummary,
so for aliases we create as many clones as there are aliasee clones in
the LTO backend. However, that information is only in the prevailing
symbol's summary, as we don't keep the memprof summary information for
other copies (to reduce memory and compile time).
In the case of weak aliases, it is possible that the prevailing copy
of the alias may be in a different module than the prevailing copy of
the aliasee (e.g. when a module with a weak_odr aliasee definition does
not have a def of the weak_odr alias and is listed first on the link
line). In that case, we were not creating the expected clones of the
alias.
Rather than a more complex solution that adds additional summary
information, detect this case and simply don't add the callsites in the
aliasee function to the callsite context graph. This will result in
conservativeness (because we can't clone through that function), but
this should be a corner case.
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