scylla-gdb.py: add line information to coroutine names in scylla fiber

For convenience.

Note that this line info only points to the function as a whole, not to the
current suspend point.
I think there's no facility for converting the `__coro_index` to the current suspend point automatically.

Before:

```
(gdb) scylla fiber seastar::local_engine->_current_task
[shard  1] #0  (task*) 0x0000601008e8e970 0x000000000047aae0 vtable for seastar::internal::coroutine_traits_base<void>::promise_type + 16  (.resume is seastar::future<void> sstables::parse<unsigned int, std::pair<sstables::metadata_type, unsigned int> >(schema const&, sstables::sstable_version_types, sstables::random_access_reader&, sstables::disk_array<unsigned int, std::pair<sstables::metadata_type, unsigned int> >&) [clone .resume] )
[shard  1] #1  (task*) 0x00006010092acf10 0x000000000047aae0 vtable for seastar::internal::coroutine_traits_base<void>::promise_type + 16  (.resume is sstables::parse(schema const&, sstables::sstable_version_types, sstables::random_access_reader&, sstables::statistics&) [clone .resume] )
[shard  1] #2  (task*) 0x0000601008e648d0 0x000000000047aae0 vtable for seastar::internal::coroutine_traits_base<void>::promise_type + 16  (.resume is sstables::sstable::read_simple<(sstables::component_type)8, sstables::statistics>(sstables::statistics&)::{lambda(sstables::sstable_version_types, seastar::file&&, unsigned long)#1}::operator()(sstables::sstable_version_types, seastar::file&&, unsigned long) const [clone .resume] )
```

After:

```
(gdb) scylla fiber seastar::local_engine->_current_task
[shard  1] #0  (task*) 0x0000601008e8e970 0x000000000047aae0 vtable for seastar::internal::coroutine_traits_base<void>::promise_type + 16  (sstables::parse<unsigned int, std::pair<sstables::metadata_type, unsigned int> >(schema const&, sstables::sstable_version_types, sstables::random_access_reader&, sstables::disk_array<unsigned int, std::pair<sstables::metadata_type, unsigned int> >&) at sstables/sstables.cc:352)
[shard  1] #1  (task*) 0x00006010092acf10 0x000000000047aae0 vtable for seastar::internal::coroutine_traits_base<void>::promise_type + 16  (sstables::parse(schema const&, sstables::sstable_version_types, sstables::random_access_reader&, sstables::statistics&) at sstables/sstables.cc:570)
[shard  1] #2  (task*) 0x0000601008e648d0 0x000000000047aae0 vtable for seastar::internal::coroutine_traits_base<void>::promise_type + 16  (sstables::sstable::read_simple<(sstables::component_type)8, sstables::statistics>(sstables::statistics&)::{lambda(sstables::sstable_version_types, seastar::file&&, unsigned long)#1}::operator()(sstables::sstable_version_types, seastar::file&&, unsigned long) const at sstables/sstables.cc:992)

```

Closes scylladb#19478

scylla-gdb.py

@@ -3867,11 +3867,11 @@ def _probe_pointer(self, ptr, scanned_region_size, using_seastar_allocator, verb
         # So if the task is a coroutine, we should be able to find the resume function via offsetting by -2.
         # AFAIK both major compilers respect this convention.
         if resolved_symbol.startswith('vtable for seastar::internal::coroutine_traits_base'):
-            coroutine_resume_fn = resolve((gdb.Value(ptr).cast(self._vptr_type) - 2).dereference())
-            if coroutine_resume_fn:
-                resolved_symbol += f" (.resume is {coroutine_resume_fn})"
+            if block := gdb.block_for_pc((gdb.Value(ptr).cast(self._vptr_type) - 2).dereference()):
+                resume = block.function
+                resolved_symbol += f" ({resume.print_name} at {resume.symtab.filename}:{resume.line})"
             else:
-                resolved_symbol += f" (.resume is unknown)"
+                resolved_symbol += f" (unknown coroutine)"
 
         if using_seastar_allocator:
             if ptr_meta is None: