linking: Replace assert(...) by macros from invariant.h

While new/modified code shouldn't use assert(...) anyway, we had several
existing uses of assert. Once we remove nonstd/optional.h, we will no
longer implicitly include cassert, implying that a lot of #include
<cassert> changes will be necessary. Instead of adding these, go all the
way and fix the undesired uses of assert.

Author: tautschnig

src/linking/linking.cpp

@@ -1215,7 +1215,7 @@ void linkingt::duplicate_type_symbol(
   symbolt &old_symbol,
   const symbolt &new_symbol)
 {
-  assert(new_symbol.is_type);
+  PRECONDITION(new_symbol.is_type);
 
   if(!old_symbol.is_type)
   {
@@ -1310,7 +1310,7 @@ bool linkingt::needs_renaming_type(
   const symbolt &old_symbol,
   const symbolt &new_symbol)
 {
-  assert(new_symbol.is_type);
+  PRECONDITION(new_symbol.is_type);
 
   if(!old_symbol.is_type)
     return true;