New loplugin:external

...warning about (for now only) functions and variables with external linkage
that likely don't need it.

The problems with moving entities into unnamed namespacs and breaking ADL
(as alluded to in comments in compilerplugins/clang/external.cxx) are
illustrated by the fact that while

  struct S1 { int f() { return 0; } };
  int f(S1 s) { return s.f(); }
  namespace N {
    struct S2: S1 { int f() { return 1; } };
    int f(S2 s) { return s.f(); }
  }
  int main() { return f(N::S2()); }

returns 1, both moving just the struct S2 into an nunnamed namespace,

  struct S1 { int f() { return 0; } };
  int f(S1 s) { return s.f(); }
  namespace N {
    namespace { struct S2: S1 { int f() { return 1; } }; }
    int f(S2 s) { return s.f(); }
  }
  int main() { return f(N::S2()); }

as well as moving just the function f overload into an unnamed namespace,

  struct S1 { int f() { return 0; } };
  int f(S1 s) { return s.f(); }
  namespace N {
    struct S2: S1 { int f() { return 1; } };
    namespace { int f(S2 s) { return s.f(); } }
  }
  int main() { return f(N::S2()); }

would each change the program to return 0 instead.

Change-Id: I4d09f7ac5e8f9bcd6e6bde4712608444b642265c
Reviewed-on: https://gerrit.libreoffice.org/60539
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

cppu/qa/cppumaker/test_cppumaker.cxx

@@ -367,7 +367,7 @@
 
 namespace test { namespace codemaker { namespace cppumaker {
 
-bool operator ==(
+static bool operator ==(
     test::codemaker::cppumaker::TestException1 const & e1,
     test::codemaker::cppumaker::TestException1 const & e2)
 {