[DSLX] IR conversion fails to resolve function invocations inside new-style proc next() method

**Describe the bug**

Consider the following code that calls some `std::-`function in next() in new-style proc

```Rust
import std;
#[test]
proc MinimalProcTest {
done: chan out,
}
impl MinimalProcTest {
fn new(done: chan out) -> Self {
MinimalProcTest { done }
}
fn next(self) {
let x = std::abs(s32:-42);
send(join(), self.done, x == s32:42);
}
}
```

```
$ bazel-bin/xls/dslx/interpreter_main fun_in_next.x --evaluator=ir-interpreter
Error: INTERNAL: IrConversionError: fun_in_next.x:11:25-11:34 Could not find name for invocation: `__std__abs__32`; available: []
=== Source Location Trace: ===
xls/dslx/ir_convert/function_converter.cc:113
xls/dslx/ir_convert/function_converter.cc:999
xls/dslx/ir_convert/function_converter.cc:678
xls/dslx/ir_convert/function_converter.cc:5053
xls/dslx/ir_convert/function_converter.cc:5013
xls/dslx/ir_convert/function_converter.cc:4326
xls/dslx/ir_convert/ir_converter.cc:365
xls/dslx/ir_convert/ir_converter.cc:486
xls/dslx/run_routines/ir_test_runner.cc:237
xls/dslx/run_routines/run_routines.cc:1031
xls/dslx/interpreter_main.cc:280
```

**Expected behavior**
No crash :)

**Environment (this can be helpful for troubleshooting):**
built xls from head (also with some local patches applied #4909 and #4292

**Additional context**

NB: AI suggested patch

```patch
--- a/xls/dslx/get_conversion_records.cc
+++ b/xls/dslx/get_conversion_records.cc
@@ -420,6 +420,12 @@ class ConversionRecordVisitor : public AstNodeRecursiveVisitor {
<< canonical_initializer.constructor_type_info->name()
<< " and next() TI " << canonical_initializer.next_type_info->name();

+ ConversionRecordVisitor next_fn_visitor(
+ (*next_fn)->owner(), canonical_initializer.next_type_info,
+ include_tests_, proc_id_factory_, top_, resolved_proc_alias_,
+ records_, processed_invocations_);
+ XLS_RETURN_IF_ERROR((*next_fn)->Accept(&next_fn_visitor));
+
XLS_ASSIGN_OR_RETURN(
ConversionRecord cr,
MakeConversionRecord(p, proc_id_factory_->CreateProcId(p),
```

(we then run into the next unimplemented issue, but at least we can keep going one more)