`demangle`: Implement parsing for simplest constrained template arguments

C++20 introduced constraints, where concepts such as `std::integral` can be used to restrict template parameters:

```
template <std::integral T>
int foo(T t);  // Only accepts integral types.
```

Clang mangles these starting with a prefix `Tk`, followed by the concept name, which mangles as a type.
For example, compare `foo` and `bar` here: https://godbolt.org/z/Kzbrrchde

Note that this implementation doesn't cover the more complex case where the concept is templated too. See [llvm's implementation](https://github.com/llvm/llvm-project/commit/4b163e343cfa54c8d55c9da73c70d58f55ea9df2) and its tests for a thorough version.

PiperOrigin-RevId: 604976260
Change-Id: Ic116c5f6f27c3f7714638bdee8de11dce871f0be

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