[cxx-interop] Constructor Span(baseAddress, count) is deprecated

This constructor is unsafe, as it allows creating a `std::span` with a count higher than the size of the sequence of objects it refers to. Therefore, when hardening is enabled, an out-of-bounds access won't trap.
We make it deprecated to discourage its use.

Author: susmonteiro

lib/ClangImporter/ClangDerivedConformances.cpp

@@ -1218,6 +1218,10 @@ void swift::conformToCxxSpanIfNeeded(ClangImporter::Implementation &impl,
       impl.importDecl(constructorDecl, impl.CurrentVersion);
   if (!importedConstructor)
     return;
+
+  auto attr = AvailableAttr::createPlatformAgnostic(importedConstructor->getASTContext(), "use 'init(_:)' instead.", "", PlatformAgnosticAvailabilityKind::Deprecated);
+  importedConstructor->getAttrs().add(attr);
+
   decl->addMember(importedConstructor);
 
   impl.addSynthesizedTypealias(decl, ctx.Id_Element,

test/Interop/Cxx/stdlib/use-std-span-typechecker.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-experimental-cxx-interop -Xcc -std=c++20 2>&1
+
+// FIXME swift-ci linux tests do not support std::span
+// UNSUPPORTED: OS=linux-gnu
+
+import StdSpan
+
+let arr: [Int32] = [1, 2, 3]
+arr.withUnsafeBufferPointer { ubpointer in
+    let _ = ConstSpan(ubpointer) // okay
+    let _ = ConstSpan(ubpointer.baseAddress!, ubpointer.count) 
+    // expected-warning@-1 {{'init(_:_:)' is deprecated: use 'init(_:)' instead.}}
+}