[Sema][CTAD] Allow user defined conversion for copy-list-initialization (#94752)

Fixes #62925.

The following code:
```cpp
#include <map>

int main() {
   std::map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
   std::map m2(m1.begin(), m1.end()); // guide #1
}
```
Is rejected by clang, but accepted by both gcc and msvc:
https://godbolt.org/z/6v4fvabb5 .

So basically CTAD with copy-list-initialization is rejected.

Note that this exact code is also used in a cppreference article:
https://en.cppreference.com/w/cpp/container/map/deduction_guides

I checked the C++11 and C++20 standard drafts to see whether suppressing
user conversion is the correct thing to do for user conversions. Based
on the standard I don't think that it is correct.

```
13.3.1.4 Copy-initialization of class by user-defined conversion [over.match.copy]
Under the conditions specified in 8.5, as part of a copy-initialization of an object of class type, a user-defined
conversion can be invoked to convert an initializer expression to the type of the object being initialized.
Overload resolution is used to select the user-defined conversion to be invoked
```
So we could use user defined conversions according to the standard.

```
If a narrowing conversion is required to initialize any of the elements, the
program is ill-formed.
```
We should not do narrowing.

```
In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.
```
We should not use explicit constructors.

1 files changed, 6 insertions, 0 deletions

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index bbbc043..0f958d2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -207,6 +207,12 @@ C++20 Feature Support
   to update the ``__cpp_concepts`` macro to `202002L`. This enables
   ``<expected>`` from libstdc++ to work correctly with Clang.
 
+- User defined constructors are allowed for copy-list-initialization with CTAD.
+  The example code for deduction guides for std::map in
+  (`cppreference <https://en.cppreference.com/w/cpp/container/map/deduction_guides>`_)
+  will now work.
+  (#GH62925).
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^