Remove multiple function assertion

This commit removes an assertion that triggers when multiple functions
get mangled to the same name when using the --export-file-local-symbols
flag to goto-cc.

This assertion was previously valid when compiling files one at a time, but
commit fcd1083 introduced the possibility of compiling and linking several
files at once. This is a problem when two source files both include an
implementation file, like [1], because the implementation may contain static
functions that get mangled twice.

This commit replaces the assertion with a debug message in case users would
like to check how often this happens.

This commit fixes #5380.

[1] https://github.com/awslabs/aws-c-common/blob/master/include/aws/common/atomics_gnu.inl

Author: karkhaz

src/goto-programs/name_mangler.h

@@ -121,10 +121,9 @@ class function_name_manglert
 
       auto inserted = model.goto_functions.function_map.emplace(
         pair.second, std::move(found->second));
-      INVARIANT(
-        inserted.second,
-        "The mangled name '" + std::string(pair.second.c_str()) +
-          "' should not already exist in the codebase");
+      if(!inserted.second)
+        log.debug() << "Found a mangled name that already exists: "
+                    << std::string(pair.second.c_str()) << log.eom;
 
       model.goto_functions.function_map.erase(found);
     }