[Sanitizer][Docs] Improve docs on building Asan (#68636)

Author: boomanaiden154

clang/docs/AddressSanitizer.rst

@@ -26,7 +26,16 @@ Typical slowdown introduced by AddressSanitizer is **2x**.
 How to build
 ============
 
-Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>`_.
+Build LLVM/Clang with `CMake <https://llvm.org/docs/CMake.html>` and enable
+the ``compiler-rt`` runtime. An example CMake configuration that will allow
+for the use/testing of AddressSanitizer:
+
+.. code-block:: console
+
+   cmake -DCMAKE_BUILD_TYPE=Release \
+     -DLLVM_ENABLE_PROJECTS="clang" \
+     -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
+     ../llvm
 
 Usage
 =====