[llvm] [ProgrammersManual] Update report_fatal_error docs (PR #138502)

[Mehdi Amini via llvm-commits]

================
@@ -437,6 +437,15 @@ that should never be entered if the program invariants hold:
       llvm_unreachable("X should be Foo or Bar here");
   }
 
+Additionally, ``reportFatalInternalError`` can be used to report invariant
+violations even in builds that do not enable assertions:
+
+.. code-block:: c++
+
+  if (VerifyFooAnalysis && !Foo.verify()) {
+    reportFatalInternalError("Analysis 'foo' not preserved");
+  }
+
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joker-eph wrote:

This is in a section about assertions, if you want to include this I don't think it should single out this API but be explicit around line 419.

https://github.com/llvm/llvm-project/pull/138502