clang serializes host pointer value into module binary (.pcm)

Two builds of the seastar C++20 module give me slightly different .pcms (attached).

llvm-bcanalyzer (and Claude) tell me that a host pointer value was serialized, so the build is not reproducible. This in turn reduces sccache effectiveness.

See UnknownCode171 with the funky op7; it's hex value is clearly a pointer:

```diff
--- /tmp/old.txt 2026-06-28 22:22:49.216109863 +0300
+++ /tmp/new.txt 2026-06-28 22:22:44.866024173 +0300
@@ -3550,10 +3550,10 @@

blob data = unprintable, 14112 bytes.

-
+

blob data = ''
-
+



@@ -2600477,7 +2600477,7 @@



-
+



@@ -2600498,7 +2600498,7 @@


blob data = unprintable, 20 bytes.
-
+
```

The pcms are available in https://scratch.scylladb.com/pcms.tar.gz.

Start by downloading pcms.tar.gz and comparing the two seastar C++20 module .pcms with llvm-bcanalyzer. Investigate the UnknownCode171 op7 value and the surrounding AST records to identify where the host pointer is serialized. Done means repeated builds produce identical module binaries without host pointer values.

Written by the indexing model from the issue text.