[PGO] Change MapRegionCounters to be a RecursiveASTVisitor.

This avoids the overhead of specifying all the traversal code when using
ConstStmtVisitor and makes it a lot easier to maintain this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206039 91177308-0d34-0410-b5e6-96231b3b80d8