[IRBuilder] Avoid out-of-line dtor for IRBuilderFolder (NFC) (#229803)
The virtual dtor was serving double duty as an out-of-line anchor
function. However, this means that every IRBuilder destructor would have
to call an empty out-of-line function (without LTO).
Define the virtual dtor inline and use a separate anchor method instead.
Based on discussion in https://github.com/llvm/llvm-project/pull/229437.