[clang] [clang][docs] Add docs for CCC_OVERRIDE_OPTIONS. (PR #142396)

[Abid Qadeer via cfe-commits]

https://github.com/abidh created https://github.com/llvm/llvm-project/pull/142396

As was noted in https://github.com/llvm/llvm-project/pull/140556#discussion_r2118160690, there is no documentation for `CCC_OVERRIDE_OPTIONS`. This adds the missing documentation. The information is duplicate of what we have for `FCC_OVERRIDE_OPTIONS` in flang. Once this goes in and available at https://clang.llvm.org/docs/UsersManual.html then the flang documentation can be changed to refer to it.

>From b2171eec20e356d4e37920e4b6c740a3c3a4b239 Mon Sep 17 00:00:00 2001
From: Abid Qadeer <haqadeer at amd.com>
Date: Mon, 2 Jun 2025 12:44:37 +0100
Subject: [PATCH] [clang][docs] Add docs for CCC_OVERRIDE_OPTIONS.

As was noted in https://github.com/llvm/llvm-project/pull/140556#discussion_r2118160690,
there is no documentation for `CCC_OVERRIDE_OPTIONS`. This adds the
missing documentation. The information is duplicate of what we have for
`FCC_OVERRIDE_OPTIONS` in flang. Once this goes in and available at
https://clang.llvm.org/docs/UsersManual.html then the flang
documentation can be changed to refer to it.
---
 clang/docs/UsersManual.rst | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index eb9a812f0c1c9..8c72f95b94095 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -3743,6 +3743,35 @@ Doxygen-style comments and ignores ordinary comments starting with ``//`` and
   ``-fcomment-block-commands=foo -fcomment-block-commands=bar`` does the same
   as above.
 
+.. _ccc-override-options:
+
+CCC_OVERRIDE_OPTIONS
+--------------------
+The environment variable ``CCC_OVERRIDE_OPTIONS`` can be used to edit clang's
+command line arguments. The value of this variable is a space-separated list of
+edits to perform. The edits are applied in the order in which they appear in
+``CCC_OVERRIDE_OPTIONS``. Each edit should be one of the following forms:
+
+- ``#``: Silence information about the changes to the command line arguments.
+
+- ``^FOO``: Add ``FOO`` as a new argument at the beginning of the command line
+  right after the name of the compiler executable.
+
+- ``+FOO``: Add ``FOO`` as a new argument at the end of the command line.
+
+- ``s/XXX/YYY/``: Substitute the regular expression ``XXX`` with ``YYY`` in the
+  command line.
+
+- ``xOPTION``: Removes all instances of the literal argument ``OPTION``.
+
+- ``XOPTION``: Removes all instances of the literal argument ``OPTION``, and the
+  following argument.
+
+- ``Ox``: Removes all flags matching ``O`` or ``O[sz0-9]`` and adds ``Ox`` at
+  the end of the command line.
+
+This environment variable does not affect the options added by the config files.
+
 .. _c:
 
 C Language Features