[AST] Various micro-optimizations in CXXInheritance

1. Pack std::pair<bool, unsigned> in CXXBasePaths::ClassSubobjects.
2. Use a SmallPtrSet instead of a SmallDenseSet for CXXBasePaths::VisitedDependentRecords.
3. Reorder some members of CXXBasePaths to save 8 bytes.
4. Use a SmallSetVector instead of a SetVector in CXXBasePaths::ComputeDeclsFound to avoid some allocations.

This speeds up an -fsyntax-only on all of Boost by approx 0.15%,
mainly by speeding up CXXBasePaths::lookupInBases by
approx 10%. No functional changes.

Patch by Bruno Ricci!

Differential Revision: https://reviews.llvm.org/D49302

llvm-svn: 337607