Update Clang extension criteria (llvm#96532)

This updates Clang's extension criteria to explicitly mention impacts on
other projects within the monorepo.

These changes were discussed in the following RFC:
https://discourse.llvm.org/t/rfc-require-discussion-of-impact-to-monorepo-stakeholders-when-adding-new-clang-extensions/79613

Author: AaronBallman

clang/www/get_involved.html

@@ -126,6 +126,12 @@ <h2 id="criteria">Contributing Extensions to Clang</h2>
   extension is not broken by ongoing maintenance in Clang. The test suite
   should be complete enough that another compiler vendor could conceivably
   validate their implementation of the feature against it.</li>
+
+  <li>A support story for other impacted projects within the monorepo: If the
+  extension can impact other parts of the project (libc++, lldb, compiler-rt,
+  etc), the proposal needs to document the impact for these projects to fully
+  support the extension and what level of support is expected. The impacted
+  project communities need to agree with that plan.</li>
 </ol>
 
 </div>