[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