Deprecate IntrusiveRefCntPtr::getPtr() in favour of get()

This better aligns with other LLVM-specific and C++ standard library smart
pointer types.

In particular there are at least a few uses of intrusive refcounting in the
frontend where it's worth investigating std::shared_ptr as a more appropriate
alternative.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212366 91177308-0d34-0410-b5e6-96231b3b80d8