[lldb] Skip check for conflicting filter/synth when adding a new regex.

When adding a new synthetic child provider, we check for an existing
conflicting filter in the same category (and vice versa). This is done
by trying to match the new type name against registered formatters.

However, the new type name we're registered can also be a regex
(`type synth add -x`), and in this case the conflict check is just
wrong: it will try to match the new regex as if it was a type name,
against previously registered regexes.

See https://github.com/llvm/llvm-project/issues/57947 for a longer
explanation with concrete examples of incorrect behavior.

Differential Revision: https://reviews.llvm.org/D134570