[include-cleaner] Dont boost private headers beyond public ones

Private headers should be the last resort, even if they match the name
of a symbol. It's pretty common in umrella headers to have internal file names
that match the symbol (e.g. Eigen::Matrix, declared in private header Matrix.h,
and exposed in umbrella header Eigen/Core).

Differential Revision: https://reviews.llvm.org/D157400