[cxx-interop] Do not import partial specializations of variables

They do not get imported correctly and trigger linker errors.

rdar://149232900 / resolves swiftlang#80802

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -4638,6 +4638,11 @@ namespace {
       return result;
     }
 
+    Decl *VisitVarTemplatePartialSpecializationDecl(
+        const clang::VarTemplatePartialSpecializationDecl *decl) {
+      return nullptr;
+    }
+
     Decl *VisitImplicitParamDecl(const clang::ImplicitParamDecl *decl) {
       // Parameters are never directly imported.
       return nullptr;

test/ClangImporter/const_values_cxx.swift

@@ -55,9 +55,6 @@ func foo() {
 
   print(MyClass().class_const_int)
   print(MyClass.class_static_const_int)
-
-  // TODO: This seems to be incorrectly imported, this test here is just to check that the compiler doesn't crash.
-  print(template_gcd)
 }
 
 // Only imported as external declarations:

test/Interop/Cxx/templates/Inputs/module.modulemap

@@ -177,3 +177,8 @@ module UninstantiatableSpecialMembers {
   header "uninstantiatable-special-members.h"
   requires cplusplus
 }
+
+module VariableTemplate {
+  header "variable-template.h"
+  requires cplusplus
+}

test/Interop/Cxx/templates/Inputs/variable-template.h

@@ -0,0 +1,5 @@
+template <int N, int M>
+inline const int template_gcd = template_gcd<M, N % M>;
+
+template <int N>
+inline const int template_gcd<N, 0> = N;

test/Interop/Cxx/templates/variable-template-typechecker.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -cxx-interoperability-mode=default -I %S/Inputs
+
+import VariableTemplate
+
+let _: CInt = template_gcd // expected-error {{cannot find 'template_gcd' in scope}}