[Clang][NFC] Consolidate tests for default argument substitution (#105617)

Follow-up on 8ac140f.

The test `SemaTemplate/default-parm-init.cpp` was introduced since the
fix #80288 and mainly did the following things:

- Ensure the default arguments are properly substituted inside either
the primary template & their explicit / out-of-line specializations.
- Ensure the strategy doesn't mess up the substitution of a lambda
expression as a default argument.

The 1st is for the bug of #68490, yet it does some redundant work: each
of the member functions is duplicated twice for the `sizeof` and
`alignof` operators, respectively, and the principle under the hood are
essentially the same. So this patch removes the duplication and reduces
the 8 functions to 4 functions that reveal the same thing.

The 2nd is presumably testing that the fix in #80288 doesn't impact a
complicated substitution. However, that seems unnecessary & unrelated to
the original issue. And more importantly, we don't have any problems
with that ever. Hence, I'll remove that test from this patch.

The test for default arguments is merged into
`SemaTemplate/default-arguments.cpp` with a new namespace, and hopefully
this could reduce the entropy of our testing cases.

Author: zyn0217

clang/test/SemaTemplate/default-arguments.cpp

@@ -229,3 +229,55 @@ namespace unevaluated {
   template<int = 0> int f(int = a); // expected-warning 0-1{{extension}}
   int k = sizeof(f());
 }
+
+#if __cplusplus >= 201103L
+namespace GH68490 {
+
+template <typename T> struct S {
+  template <typename U>
+  constexpr int SizeOfU(int param = sizeof(U)) const;
+
+  template <typename U>
+  constexpr int SizeOfT(int param = sizeof(T)) const;
+};
+
+template <typename T> struct S<T *> {
+  template <typename U>
+  constexpr int SizeOfU(int param = sizeof(U)) const;
+
+  template <typename U>
+  constexpr int SizeOfT(int param = sizeof(T *)) const;
+};
+
+template <typename T>
+template <typename U>
+constexpr int S<T *>::SizeOfU(int param) const {
+  return param;
+}
+
+template <typename T>
+template <typename U>
+constexpr int S<T *>::SizeOfT(int param) const {
+  return param;
+}
+
+template <>
+template <typename T>
+constexpr int S<int>::SizeOfU(int param) const {
+  return param;
+}
+
+template <>
+template <typename T>
+constexpr int S<int>::SizeOfT(int param) const {
+  return param;
+}
+
+static_assert(S<int>().SizeOfU<char>() == sizeof(char), "");
+static_assert(S<int>().SizeOfT<char>() == sizeof(int), "");
+static_assert(S<short *>().SizeOfU<char>() == sizeof(char), "");
+static_assert(S<short *>().SizeOfT<char>() == sizeof(short *), "");
+
+} // namespace GH68490
+
+#endif