[clang] [llvm] [Clang][AIX] Add -mloadtime-comment-vars flag to preserve identifying variables (PR #187986)

[Hubert Tong via cfe-commits]

================
@@ -6873,6 +6873,100 @@ When ``#pragma comment(copyright, ...)`` appears in a C++20 module interface
 unit, the copyright string is embedded only in the object file compiled from
 that interface unit. Importing TUs do not re-emit the string.
 
+Preserving Identifying Variables with -mloadtime-comment-vars
+--------------------------------------------------------------
+
+The ``-mloadtime-comment-vars=`` flag accepts a comma-separated list of
+global variable names that should be preserved in the final object file as
+loadtime identifying strings. This is an AIX-specific feature; on other
+targets the compiler emits a warning and the flag is not forwarded to
+``-cc1``.
+
+This flag complements ``#pragma comment(copyright, ...)`` for codebases that
+already use the traditional UNIX convention of embedding identifying strings
+directly in source variables rather than via a pragma.
+
+Syntax:
+
+.. code-block:: console
+
+  -mloadtime-comment-vars=<var1>[,<var2>,...]
+
+Name matching:
+
+Names are matched against the variable's **mangled IR symbol name** — the
+name as it appears in the object file.
+
+- In C, file-scope static variables are not mangled, so the mangled name is
+  identical to the source identifier (for example, ``sccsid``).
+- In C++, variables are mangled using the Itanium ABI. To find the mangled
+  name, compile with ``clang -S -emit-llvm`` and look for the global in the
+  ``.ll`` output, or run ``nm`` on the object file.
----------------
hubert-reinterpretcast wrote:

This is "helpful" in a non-helpful way. If the user actually needed to use this feature, the suggestion wouldn't work unless they also changed their source (e.g., to add `__attribute__((__used__))`).
```suggestion
```

https://github.com/llvm/llvm-project/pull/187986