Linking: shadowed symbols should not trigger warnings

The Unix linker silently ignores (re-)definitions when these are already
present. We may even internally generate multiple definitions for GCC's
built-ins, warnings about which would always be spurious.

This avoids spurious build failures when linking with -Werror.

Author: tautschnig

regression/goto-cc-file-local/name-clash-with-suffix/test.desc

@@ -4,7 +4,7 @@ use_find out-file-counter final-link wall suffix assertion-check
 ^EXIT=0$
 ^SIGNAL=0$
 --
-^.*warning: function '__CPROVER_file_local_main_c_static_fun' in module 'main' is shadowed by a definition in module 'main'
+^.*: function '__CPROVER_file_local_main_c_static_fun' in module 'main' is shadowed by a definition in module 'main'
 ^warning: ignoring
 ^\*\*\*\* WARNING: no body for function
 --

regression/goto-cc-file-local/name-clash/test.desc

@@ -3,7 +3,7 @@ foo/bar/baz/main.c
 use_find out-file-counter final-link wall
 ^EXIT=0$
 ^SIGNAL=0$
-^.*warning: function '__CPROVER_file_local_main_c_static_fun' in module 'main' is shadowed by a definition in module 'main'
+^.*: function '__CPROVER_file_local_main_c_static_fun' in module 'main' is shadowed by a definition in module 'main'
 --
 ^warning: ignoring
 ^\*\*\*\* WARNING: no body for function

src/linking/linking.cpp

@@ -796,12 +796,12 @@ void linkingt::duplicate_code_symbol(
     else if(base_type_eq(old_symbol.type, new_symbol.type, ns))
     {
       // keep the one in old_symbol -- libraries come last!
-      warning().source_location=new_symbol.location;
+      debug().source_location = new_symbol.location;
 
-      warning() << "function '" << old_symbol.name << "' in module '"
-                << new_symbol.module
-                << "' is shadowed by a definition in module '"
-                << old_symbol.module << "'" << eom;
+      debug() << "function '" << old_symbol.name << "' in module '"
+              << new_symbol.module
+              << "' is shadowed by a definition in module '"
+              << old_symbol.module << "'" << eom;
     }
     else
       link_error(