[Clang][NFCI] Cleanup the fix for default function argument substitution (#104911)

(This is one step towards tweaking `getTemplateInstantiationArgs()` as
discussed in https://github.com/llvm/llvm-project/pull/102922)

We don't always substitute into default arguments while transforming a
function parameter. In that case, we would preserve the uninstantiated
expression until after, e.g. building up a CXXDefaultArgExpr and
instantiate the expression there.

For member function instantiation, this algorithm used to cause a
problem in that the default argument of an out-of-line member function
specialization couldn't get properly instantiated. This is because, in
`getTemplateInstantiationArgs()`, we would give up visiting a function's
declaration context if the function is a specialization of a member
template. For example,

```cpp
template <class T>
struct S {
  template <class U>
  void f(T = sizeof(T));
};

template <> template <class U>
void S<int>::f(int) {}
```

The default argument `sizeof(U)` that lexically appears inside the
declaration would be copied to the function declaration in the class
template specialization `S<int>`, as well as to the function's
out-of-line definition. We use template arguments collected from the
out-of-line function definition when substituting into the default
arguments. We would therefore give up the traversal after the function,
resulting in a single-level template argument of the `f` itself. However
the default argument here could still reference the template parameters
of the primary template, hence the error.

In fact, this is similar to constraint checking in some respects: we
actually want the "whole" template arguments relative to the primary
template, not those relative to the function definition. So this patch
adds another flag to indicate `getTemplateInstantiationArgs()` for that.

This patch also consolidates the tests for default arguments and removes
some unnecessary tests.