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

[Boaz Brickner via cfe-commits]

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+============================
+Warning suppression mappings
+============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+Warning suppression mappings enable 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 some headers.
+
+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 in large codebases can be
+difficult today. Since it requires cleaning up all the existing warnings,
+which might not be possible when some dependencies aren't in the project owner's
+control or because new violations are creeping up quicker than the clean up.
+
+Warning suppression mappings aim to alleviate some of these concerns by making
+diagnostic configuration granularity finer, at a source file level.
+
+To achieve this, user can create a file that lists which diagnostic groups to
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bricknerb wrote:

Perhaps refer somewhere to what is a "diagnostic group" so a user can easily what diagnostic groups are available?

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