Block coverage reporting: do not use built-in locations

source_linest::insert will disregard any source locations that refer to
built-ins. cover_basic_blockst, however, only checked that any source
location used as basic-block representative had file and line
information. This resulted in cover_basic_blockst believing it had found
a representative source location when source_linest didn't quite agree.

This change makes the example presented in #6536 (which originates from
Rust/Kani) work. The issue wasn't reproducible with C examples.

Fixes: #6536

Author: tautschnig

src/goto-instrument/cover_basic_blocks.cpp

@@ -65,6 +65,7 @@ cover_basic_blockst::cover_basic_blockst(const goto_programt &goto_program)
       !it->source_location().is_nil() &&
       !it->source_location().get_file().empty() &&
       !it->source_location().get_line().empty() &&
+      !it->source_location().is_built_in() &&
       block_info.source_location.is_nil())
     {
       block_info.representative_inst = it; // update