[libcxxabi] [llvm] [WIP] [libcxxabi][ItaniumDemangle] Add infrastructure to track location information of parts of a demangled function name (PR #133249)

[Michael Buch via llvm-commits]

Michael137 wrote:

> This approach seems very specific to this particular use case. That doesn't seem like a great fit here; the demangler has generally been designed to be pretty neutral to its use case, including things like the [fuzzy matcher](https://github.com/llvm/llvm-project/blob/main/llvm/lib/ProfileData/ItaniumManglingCanonicalizer.cpp) that's used for profile info being implemented outside the demangler through a few hooks. I'd imagine other users of the demangler might want something similar to this, but different -- for example, full syntax highlighting of demangled names, or eliding template argument lists and function parameter types when printing demangled names -- so I think we shouldn't be adding something this coupled to the lldb use case here. How would you feel about a somewhat different approach:
> 
> * Instead of having each `Node::print*` function call other `Node::print*` functions, make it call a new `OutputStream::print*(Node*)` function.
> * Make that new function virtual, with a default implementation that just calls back into the `Node`s, and ensure that clients of the demangler can pass in their own custom `OutputStream`.
> * Override this part of `OutputStream` from lldb and add a small state machine to track which parts of the output correspond to which nodes.
> 
> You'll also need to add hooks to `OutputBuffer::insert` and `OutputBuffer::prepend`, which can insert text at a position that's not the end of the buffer, and `OutputBuffer::setCurrentPosition`, which effectively erases from the new current position to the old end of the buffer.

Yea I agree this would be very specific to how LLDB uses this API. Happy to make it more generic as you propose. Let me give that a shot!

https://github.com/llvm/llvm-project/pull/133249