Add LLVM bug no. for clang-cl static_assert issue

BUG=aomedia:2875

Change-Id: Idb321de5511991ba3394db0cbeb6c723fc7d71ab
(cherry picked from commit 0690e68dc423e677b2b96676f580e7358788124b)
diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index fe11b41..ea7b37c 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -3117,7 +3117,8 @@
 
 #if __STDC_VERSION__ >= 201112L
   // We use the keyword _Static_assert because clang-cl does not allow the
-  // convenience macro static_assert to be used in function scope.
+  // convenience macro static_assert to be used in function scope. See
+  // https://bugs.llvm.org/show_bug.cgi?id=48904.
   _Static_assert(sizeof(ctx->cpi->common.error.detail) >= ARG_ERR_MSG_MAX_LEN,
                  "The size of the err_msg buffer for arg_match_helper must be "
                  "at least ARG_ERR_MSG_MAX_LEN");