Uses the `TrackingOutputBuffer` to populate the new member `Mangled::m_demangled_info`.

`m_demangled_info` is lazily popluated by `GetDemangledInfo`. To ensure `m_demangled` and `m_demangled_info` are in-sync we clear `m_demangled_info` anytime `m_demangled` is set/cleared.

https://github.com/llvm/llvm-project/pull/131836

This patch implements a new `TrackingOutputBuffer` which tracks where the scope/basename/arguments begin and end in the demangled string.

The idea that a function name can be decomposed into <scope, base, arguments>. The assumption is that given the ranges of those three elements and the demangled name, LLDB will be able to to reconstruct the full demangled name. The tracking of those ranges is pretty simple. We don’t ever deal with nesting, so whenever we recurse into a template argument list or another function type, we just stop tracking any positions. Once we recursed out of those, and are back to printing the top-level function name, we continue tracking the positions.

We introduce a new structure `FunctionNameInfo` that holds all this information and is stored in the new `TrackingOutputBuffer` class.

Tests are in `ItaniumDemangleTest.cpp`.

https://github.com/llvm/llvm-project/pull/131836