[llvm] [docs] Refresh Developer Policy text (PR #136198)

[Aaron Ballman via llvm-commits]

================
@@ -220,27 +219,47 @@ Test Cases
 Developers are required to create test cases for any bugs fixed and any new
 features added.  Some tips for getting your testcase approved:
 
-* All feature and regression test cases are added to the ``llvm/test``
-  directory. The appropriate sub-directory should be selected (see the
+* All feature and regression test cases are added to the ``test`` subdirectory
+  of each LLVM subproject, i.e. ``llvm-project/llvm/test`` for LLVM itself. The
+  appropriate sub-directory should be selected (see the
   :doc:`Testing Guide <TestingGuide>` for details).
 
-* Test cases should be written in :doc:`LLVM assembly language <LangRef>`.
-
-* Test cases, especially for regressions, should be reduced as much as possible,
-  by :doc:`bugpoint <Bugpoint>` or manually. It is unacceptable to place an
-  entire failing program into ``llvm/test`` as this creates a *time-to-test*
-  burden on all developers. Please keep them short.
+* We prefer that functional changes are tested using ``FileCheck`` and the tool
+  that fits most closely with the code being modified. For example, ``opt`` is
+  used to test IR transformations, ``llc`` for backend changes, and ``clang``
+  for frontend changes. Some components have scripts for generating and
+  updating golden tests in the ``utils/`` subproject directory, i.e.
+  `mlir/utils/generate-test-checks.py <https://github.com/llvm/llvm-project/blob/main/mlir/utils/generate-test-checks.py>`_
+  and `llvm/utils/update_llc_test_checks.py <https://github.com/llvm/llvm-project/blob/main/llvm/utils/update_llc_test_checks.py>`_
+
+* Some subprojects such as ``clang`` and ``clangd`` have project specific
+  testing tools, like the ``clang -verify`` flag (`docs
+  <https://clang.llvm.org/docs/InternalsManual.html#verifying-diagnostics>`_)
+  and the ``clangd -lit-test``
+  flag, which are preferred over FileCheck.
----------------
AaronBallman wrote:

```suggestion
  flag, which are preferred over ``FileCheck``.
```

https://github.com/llvm/llvm-project/pull/136198