Add a comment to TempRValue optimization.

A recent fix added some code that is inconsistent with the design of
the pass, creates an implicit coupling between separate parts of the
optimization, and relies on incredibly subtle reasoning to ensure
correctness.

This sort of thing needs a big fat comment.

lib/SILOptimizer/Transforms/CopyForwarding.cpp

@@ -1659,8 +1659,18 @@ bool TempRValueOptPass::collectLoads(
 
     // Check if there is another function argument, which is inout which might
     // modify the source of the copy_addr.
-    // If we would remove the copy_addr in this case, it would result in an
-    // exclusivity violation.
+    //
+    // When a use of the temporary is an apply, then we need to prove that the
+    // function called by the apply cannot modify the temporary's source
+    // value. By design, this should be handled by
+    // `checkNoSourceModification`. However, this would be too conservative
+    // since it's common for the apply to have an @out argument, and alias
+    // analysis cannot prove that the @out does not alias with `src`. Instead,
+    // `checkNoSourceModification` always avoids analyzing the current use, so
+    // applies need to be handled here. We already know that an @out cannot
+    // alias with `src` because the `src` value must be initialized at the point
+    // of the call. Hence, it is sufficient to check specifically for another
+    // @inout that might alias with `src`.
     auto calleeConv = apply.getSubstCalleeConv();
     unsigned calleeArgIdx = apply.getCalleeArgIndexOfFirstAppliedArg();
     for (Operand &operand : apply.getArgumentOperands()) {