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

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,92 @@
+============================
+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:
----------------
AaronBallman wrote:

```suggestion
to be clean. Hence turning on new warnings in large codebases can be difficult
today:
```

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