Don't open existentials when calling C++ function templates

C++ function templates require specialization, which does not work with
opened existentials. Disable opening for them.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1354,6 +1354,11 @@ shouldOpenExistentialCallArgument(
     break;
   }
 
+  // C++ function templates require specialization, which is not possible with
+  // opened existential archetypes, so do not open.
+  if (isa_and_nonnull<clang::FunctionTemplateDecl>(callee->getClangDecl()))
+    return None;
+
   ASTContext &ctx = callee->getASTContext();
   if (!ctx.LangOpts.EnableOpenedExistentialTypes)
     return None;

test/Interop/Cxx/templates/function-template.swift

@@ -1,4 +1,5 @@
 // RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop)
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop -Xfrontend -enable-experimental-opened-existential-types)
 //
 // REQUIRES: executable_test