[llvm] [llvm-debuginfo-analyzer] Remove `LVScope::Children` container (PR #144750)

[Javier Lopez-Gomez via llvm-commits]

jalopezg-git wrote:

This PR should yield further memory footprint improvements after https://github.com/llvm/llvm-project/pull/144399 is merged.

As suggested in https://github.com/llvm/llvm-project/issues/69160, removing `LVScope::Children`.  Note, however, that children iteration order is now _(1)_ Types, _(2)_ Symbols, and finally _(3)_ Scopes.  This may be (or not) considered important - I think it's not.
`
llvm-debuginfo-analyzer` tests should be fixed though due to iteration order and `CHECK-NEXT:` FileCheck directives used in those tests.

https://github.com/llvm/llvm-project/pull/144750