Updates the Stan submodule to fa0a985.

stan-buildbot · stan-buildbot · commit d84693c22895 · 2021-01-01T14:03:45.000-05:00
diff --git a/stan b/stan
@@ -1 +1 @@
-Subproject commit 22c371660355ace9188255530af8c7e9baf5e840
+Subproject commit fa0a9851ab7ba3192bc332b658c7005527a48bfa
