[clang] Eliminate TypeProcessingState::trivial.

This flag is redundant -- it's true iff `savedAttrs` is empty.

Querying `savedAttrs.empty()` should not take any more time than querying the
`trivial` flag, so this should not have a performance impact either.

I noticed this while working on https://reviews.llvm.org/D111548.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D123783