[lldb][NFC] Allow range-based for loops over DWARFDIE's children

This patch adds the ability to get a DWARFDIE's children as an LLVM range.

This way we can use for range loops to iterate over them and we can use LLVM's
algorithms like `llvm::all_of` to query all children.

The implementation has to do some small shenanigans as the iterator needs to
store a DWARFDIE, but a DWARFDIE container is also a DWARFDIE so it can't return
the iterator by value. I just made the `children` getter a templated function to
avoid the cyclic dependency.

Reviewed By: #lldb, werat, JDevlieghere

Differential Revision: https://reviews.llvm.org/D103172