Tests/RunCMake/CXXModules: declare `forwarding` as `extern "C++"`

It is not part of any module, so it needs to be declared as such.

Tests/RunCMake/CXXModules/examples/export-bmi-and-interface-build/importable.cxx

@@ -1,6 +1,8 @@
 export module importable;
 
+extern "C++" {
 int forwarding();
+}
 
 export int from_import()
 {

Tests/RunCMake/CXXModules/examples/export-bmi-and-interface-install/importable.cxx

@@ -1,6 +1,8 @@
 export module importable;
 
+extern "C++" {
 int forwarding();
+}
 
 export int from_import()
 {

Tests/RunCMake/CXXModules/examples/export-interface-build/importable.cxx

@@ -1,6 +1,8 @@
 export module importable;
 
+extern "C++" {
 int forwarding();
+}
 
 export int from_import()
 {

Tests/RunCMake/CXXModules/examples/export-interface-install/importable.cxx

@@ -1,6 +1,8 @@
 export module importable;
 
+extern "C++" {
 int forwarding();
+}
 
 export int from_import()
 {

Tests/RunCMake/CXXModules/examples/export-interface-no-properties-build/importable.cxx

@@ -1,6 +1,8 @@
 export module importable;
 
+extern "C++" {
 int forwarding();
+}
 
 export int from_import()
 {

Tests/RunCMake/CXXModules/examples/export-interface-no-properties-install/importable.cxx

@@ -1,6 +1,8 @@
 export module importable;
 
+extern "C++" {
 int forwarding();
+}
 
 export int from_import()
 {