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

[Aaron Ballman via llvm-commits]

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@@ -220,27 +218,41 @@ 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>`.
+* We prefer that functional changes are tested using ``FileCheck`` and the tool
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AaronBallman wrote:

This is a bit awkward for the Clang side of things because we prefer `-verify` tests over `FileCheck` when testing diagnostic behavior and `FileCheck` when testing codegen output. Should we maybe mention `-verify` and link to the Clang docs on it?

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