Mark PassBuilder::addPass() as minsize

PassBuilder.cpp is the slowest LLVM file to compile (if only building X86).
This makes PassBuilder.o a little faster to compile and a little smaller
as well.

These methods are not performance critical at all but are instantiated many times.

83M -> 72M instructions according to perf stat.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D110782