[c++20] P1907R1: Support for generalized non-type template arguments of scalar type. (#78041)

Previously committed as 9e08e51a20d0d2b1c5724bb17e969d036fced4cd, and
reverted because a dependency commit was reverted, then committed again
as 4b574008aef5a7235c1f894ab065fe300d26e786 and reverted again because
"dependency commit" 5a391d38ac6c561ba908334d427f26124ed9132e was
reverted. But it doesn't seem that 5a391d38ac6c was a real dependency
for this.

This commit incorporates 4b574008aef5a7235c1f894ab065fe300d26e786 and
18e093faf726d15f210ab4917142beec51848258 by Richard Smith (@zygoloid),
with some minor fixes, most notably:

- `UncommonValue` renamed to `StructuralValue`

- `VK_PRValue` instead of `VK_RValue` as default kind in lvalue and
member pointer handling branch in
`BuildExpressionFromNonTypeTemplateArgumentValue`;

- handling of `StructuralValue` in `IsTypeDeclaredInsideVisitor`;

- filling in `SugaredConverted` along with `CanonicalConverted`
parameter in `Sema::CheckTemplateArgument`;

- minor cleanup in
`TemplateInstantiator::transformNonTypeTemplateParmRef`;

- `TemplateArgument` constructors refactored;

- `ODRHash` calculation for `UncommonValue`;

- USR generation for `UncommonValue`;

- more correct MS compatibility mangling algorithm (tested on MSVC ver.
19.35; toolset ver. 143);

- IR emitting fixed on using a subobject as a template argument when the
corresponding template parameter is used in an lvalue context;

- `noundef` attribute and opaque pointers in `template-arguments` test;

- analysis for C++17 mode is turned off for templates in
`warn-bool-conversion` test; in C++17 and C++20 mode, array reference
used as a template argument of pointer type produces template argument
of UncommonValue type, and
`BuildExpressionFromNonTypeTemplateArgumentValue` makes
`OpaqueValueExpr` for it, and `DiagnoseAlwaysNonNullPointer` cannot see
through it; despite of "These cases should not warn" comment, I'm not
sure about correct behavior; I'd expect a suggestion to replace `if` by
`if constexpr`;

- `temp.arg.nontype/p1.cpp` and `dr18xx.cpp` tests fixed.

1 files changed, 13 insertions, 5 deletions

diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
index 197726f..a48f35e 100755
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@@ -637,13 +637,21 @@ C++23, informally referred to as C++26.</p>
     </tr>
     <!-- Rapperswil papers -->
     <tr>
-      <td rowspan="2">Class types as non-type template parameters</td>
+      <td>Class types as non-type template parameters</td>
       <td><a href="https://wg21.link/p0732r2">P0732R2</a></td>
-      <td rowspan="2" class="partial" align="center">Partial</td>
+      <td class="full" align="center">Clang 12</td>
+    </tr>
+    <tr> <!-- from Belfast -->
+      <td>Generalized non-type template parameters of scalar type</td>
+      <td><a href="https://wg21.link/p1907r1">P1907R1</a></td>
+      <td class="partial" align="center">
+        <details>
+          <summary>Clang 18 (Partial)</summary>
+          Reference type template arguments referring to instantiation-dependent objects and subobjects
+          (i.e. declared inside a template but neither type- nor value-dependent) aren't fully supported.
+        </details>
+      </td>
     </tr>
-      <tr> <!-- from Belfast -->
-        <td><a href="https://wg21.link/p1907r1">P1907R1</a></td>
-      </tr>
     <tr>
       <td>Destroying operator delete</td>
       <td><a href="https://wg21.link/p0722r3">P0722R3</a></td>