[libcxxabi] [llvm] [ItaniumDemangle] Unconditionally parse substitution template arguments (PR #131970)

[Richard Smith via llvm-commits]

zygoloid wrote:

I think the point here is to handle a mangling collision between:
```c++
template<typename T> struct X {};
struct C {
  operator X<int>();
} c;
X<int> x = c;
```
and:
```c++
struct X {};
struct C {
  template<typename T = int>
  operator X() {}
} c;
X x = c;
```
where in both cases the conversion function mangles as `_ZN1Ccv1XIiEEv`. This isn't a problem for the ABI, because these can't both exist in the same program -- `X` is either a template or a non-template -- but it's a problem for the demangler because it needs to pick one of those two options.

I think that's why `TryToParseTemplateArgs` is set to `false` in `parseOperatorName` -- it's trying to pick the second interpretation above, by stopping parsing the type name at the `I`. I don't know why we set `TryToParseTemplateArgs` to `false` in `parseConvertExpr` or on `parseExpr` when parsing a conversion expression, because the same ambiguity doesn't exist there (as far as I'm aware); perhaps this was added too broadly and those `ScopedOverride`s should not be present?

Can you check that these all still demangle with this patch applied:
```
_ZN1Ccv1XIiEEv
_ZN1Ccv1XIiEIiEEv
_ZN1Ccv1XIT_EIiEEv
```

https://github.com/llvm/llvm-project/pull/131970