[clang] [clang-tools-extra] [llvm] [clang] Introduce diagnostics suppression mappings (PR #112517)

[Aaron Ballman via llvm-commits]

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+============================
+Warning suppression mappings
+============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+Warning suppression mappings enables users to suppress clang's diagnostics in a
+per-file granular manner. Enabling enforcement of diagnostics in specific parts
+of the project, even if there are violations in dependencies or other parts of
+the codebase.
+
+Goal and usage
+==============
+
+Clang allows diagnostics to be configured at a translation-unit granularity.
+If a foo.cpp is compiled with -Wfoo, all transitively included headers also need
+to be clean. Hence turning on new warnings at large codebases is quite difficult
+today:
+- It requires cleaning up all the existing warnings, which might not be possible
+  when some dependencies aren't in project owner's control.
+- Preventing backsliding in the meanwhile as the diagnostic can't be enforced at
+  all until codebase is cleaned up.
+
+Warning suppression mappings aims to alleviate some of these concerns by making
+diagnostic configuration granularity finer, at a source file level.
+
+To achieve this, user may create a file listing which diagnostic groups to
+suppress in which files, and pass it as a command line argument to clang with
+``--warning-suppression-mappings`` flag.
----------------
AaronBallman wrote:

```suggestion
To achieve this, user can create a file that lists which diagnostic groups to
suppress in which files or paths, and pass it as a command line argument to Clang with
the ``--warning-suppression-mappings`` flag.
```

https://github.com/llvm/llvm-project/pull/112517