[clang] [llvm] [clang] Add `-verify-directives` cc1 flag (PR #179835)

[Aaron Ballman via cfe-commits]

================
@@ -3451,22 +3451,38 @@ are similar.
 Testing
 -------
 All functional changes to Clang should come with test coverage demonstrating
-the change in behavior.
+the change in behavior. There are four kinds of tests:
+
+* Unit tests: such tests are placed in ``clang/test/unittest``.
+* Diagnostic tests: such tests ensures that only specific diagnostics are
+  emitted at specific source lines. Those tests are using ``-verify`` mode of
+  ``-cc1``, which is described below in
+  :ref:`"Verifying Diagnostics" <verifying-diagnostics>`. Additional
+  provisions for tests for C++ defect reports are described in ... section. 
+* AST dump tests: such tests pass AST dump to
----------------
AaronBallman wrote:

```suggestion
* AST dump tests: such tests pass AST dump to the
```

https://github.com/llvm/llvm-project/pull/179835