[NFC][VPlan] Wrap PlainCFGBuilder with an anonymous namespace.

Summary:
It's internal to the VPlanHCFGBuilder and should not be visible outside of its
translation unit.

Reviewers: dcaballe, fhahn

Reviewed By: fhahn

Subscribers: rengolin, bollu, tschuett, llvm-commits, rkruppe

Differential Revision: https://reviews.llvm.org/D47312

llvm-svn: 333187