Default template arguments added in later redeclaration not considered for CTAD

[timsong-cpp]

template<class> struct X {};
template<class=void> struct X;

X x;

Accepted by GCC and MSVC, rejected by Clang:

<source>:4:3: error: no viable constructor or deduction guide for deduction of template arguments of 'X'
    4 | X x;
      |   ^
<source>:1:24: note: candidate template ignored: couldn't infer template argument ''
    1 | template<class> struct X {};
      |                        ^
<source>:1:24: note: candidate function template not viable: requires 1 argument, but 0 were provided
    1 | template<class> struct X {};
      |                        ^
1 error generated.
Compiler returned: 1

This affects <memory_resource> in GCC 13's libstdc++, which defines polymorphic_allocator first before adding a default template argument in a later declaration.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (timsong-cpp)

```cpp template<class> struct X {}; template<class=void> struct X;

X x;

Accepted by GCC and MSVC, rejected by Clang:

<source>:4:3: error: no viable constructor or deduction guide for deduction of template arguments of 'X'
4 | X x;
| ^
<source>:1:24: note: candidate template ignored: couldn't infer template argument ''
1 | template<class> struct X {};
| ^
<source>:1:24: note: candidate function template not viable: requires 1 argument, but 0 were provided
1 | template<class> struct X {};
| ^
1 error generated.
Compiler returned: 1

This affects `<memory_resource>` in GCC 13's libstdc++, which defines `polymorphic_allocator` first before adding a default template argument in a later declaration.
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