[ModuleInliner] Remove InlineOrder::front (NFC)

InlineOrder::front is a remnant from the era when we had a nested
"while" loops in the module inliner, with the inner one grouping the
call sites with the same caller.

Now that we have a simple "while" loop draining the priority queue, we
can just use InlineOrder::pop.

Differential Revision: https://reviews.llvm.org/D134121