[ADT] Fix for ImmutableMapRef

The `Root` member of `ImmutableMapRef` was changed recently from a plain
pointer to `IntrusiveRefCntPtr`. However, the `Profile` member function
was not adjusted. This results in comilation error whenever the
`Profile` method is used on an `ImmutableMapRef`. This patch fixes this
issue and also adds unit tests for `ImmutableMapRef`.

Differential Revision: https://reviews.llvm.org/D89486

(cherry picked from commit 184eb4fa)