DomTree: Remove getRoots() accessor

Summary:
Avoid exposing details about how roots are stored. This enables subsequent
type-erasure changes.

v5:
- cleanup a unit test by using EXPECT_EQ instead of EXPECT_TRUE

Change-Id: I532b774cc71f2224e543bc7d79131d97f63f093d

Reviewers: arsenm, RKSimon, mehdi_amini, courbet

Subscribers: jvesely, wdng, hiraditya, kuhar, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83085