When inlining a `callee` with a call site debug location, the inlining
infrastructure was trivially combining the `callee` and the `caller`
locations, forming a "tree" of call stacks. Because of this, the remarks
were printing an incomplete inlining stack.
This commit handles this case and appends the `caller` location at the
end of the `callee`'s stack, extending the chain.
6b59b33358