feat: Support templated lambdas

[Swatinem]

helps with #208, but probably does not fully fix it.

This is very much WIP, as I try to wrap my head around how this parser actually works :-)
The result for lambdas is very much different from what llvm-cxxfilt

Some snippets just for my own reference:

Parser from llvm:
https://github.com/llvm/llvm-project/blob/df63eedef64d715ce1f31843f7de9c11fe1e597f/llvm/include/llvm/Demangle/ItaniumDemangle.h#L2661-L2667
https://github.com/llvm/llvm-project/blob/847299d3f00507f172097bad9dde61dfad0d355b/llvm/include/llvm/Demangle/ItaniumDemangle.h#L5388-L5442
Pretty-printer from llvm:
https://github.com/llvm/llvm-project/blob/df63eedef64d715ce1f31843f7de9c11fe1e597f/llvm/include/llvm/Demangle/ItaniumDemangle.h#L934-L986

[khuey]

The ultimate source of this construct appears to be itanium-cxx-abi/cxx-abi#85.

[khuey]

One thing that's a bit tricky here is I believe these template parameters can be used as substitutions later (see https://github.com/llvm/llvm-project/blob/847299d3f00507f172097bad9dde61dfad0d355b/llvm/include/llvm/Demangle/ItaniumDemangle.h#L5397). We may need to implement ArgScope for ClosureTypeName or Vec<TemplateParamDecl> or something. It's not immediately obvious to me how to construct a C++ symbol that would use this.

[Swatinem]

@khuey I rebased this and posted it for review.
I’m aware the feature is not fully implemented, though I won’t be putting in the effort to completely finish it up.
Considering that, I would argue to merge this as-is, even if it does not implement the feature fully, so we at least get back something rather than nothing at all (an Error).