…#82373)

It turns out there's a pathway for Functions to be inserted into modules
without having the "New" debug-info flag set correctly, which this patch
fixes. Sadly there isn't a Module::insert method to instrument out
there, everyone touches the list directly.

This fix exposes a path where such functions are produced in the
outliner in the wrong mode; requiring a fix there to correctly drop
RemoveDIs-mode debug-info. This is covered by
test/DebugInfo/AArch64/ir-outliner.ll

Function::Function's constructor sets the debug info format based on the
passed in parent Module, so by using this rather than modifying the
function list directly, we pick up the debug info format automatically.