Update TODO to note that Clang git doesn't have the POWER bug.

Change-Id: Ibe3d43440746b273eb7ddcf7235b132424ca0494
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37004
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/fipsmodule/bn/bn.c

diff --git a/crypto/fipsmodule/bn/bn.c b/crypto/fipsmodule/bn/bn.c
index c57c4e0..e3f1c90 100644
--- a/crypto/fipsmodule/bn/bn.c
+++ b/crypto/fipsmodule/bn/bn.c
@@ -393,7 +393,9 @@
   // has the same bug but this workaround is not effective there---I've not
   // been able to find a workaround for 8.0.1.
   //
-  // TODO(agl): include a bug link if possible.
+  // At the time of writing (2019-08-08), Clang git does *not* have this bug
+  // and does not need this workaroud. The current git version should go on to
+  // be Clang 10 thus, once we can depend on that, this can be removed.
   if (value_barrier_w((size_t)bn->width == words)) {
     return 1;
   }