[IPO] Opt-in local clones for thinlto imports

ThinLTO imports (which appear as `available_externally`) that survive
inlining get deleted. With today's inliner that's reasonable, because
the way the function would be inlined into in other modules would be the
same - because of the bottom-up traversal assumption, and the fact that
the inliner doesn't take into account surrounding context [*]. The
ModuleInliner invalidates the first assumption, and the ML inliner the
second.

This patch adds a way to opt-in a module to keep its variant of an
imported function, even if it survived past inlining.

[*] Almost. Deferred inlining is an exception which can lead to
(empirically) infrequent discrepancies.

Differential Revision: https://reviews.llvm.org/D150148