Check that loplugin:unreffun warns for explicitly "extern" functions

Change-Id: Ic43f007af4480b358916c2b27796a9b248ea9eb7
Reviewed-on: https://gerrit.libreoffice.org/60496
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/test/unreffun.cxx

@@ -22,4 +22,18 @@ void h() // expected-error {{Unreferenced externally visible function definition
 {
 }
 
+void i() {}
+
+extern void j(); // expected-error {{Unreferenced function declaration [loplugin:unreffun]}}
+
+extern void
+k() // expected-error {{Unreferenced externally visible function definition [loplugin:unreffun]}}
+{
+}
+
+extern void l(); // expected-note {{first declaration is here [loplugin:unreffun]}}
+void l() // expected-error {{Unreferenced externally visible function definition [loplugin:unreffun]}}
+{
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */

compilerplugins/clang/test/unreffun.hxx

@@ -12,6 +12,8 @@
 
 void f();
 
+extern void i();
+
 #endif
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */