[PassManagerBuilder] Remove PassManagerBuilder

PassManagerBuilder is dead, long live PassBuilder!

bugpoint's -O# are now useless (and probably have been for a while given the number of passes we've removed from PassManagerBuilder). Perhaps they'll be revived if bugpoint ever works with the new PM.

Reviewed By: nikic, MaskRay

Differential Revision: https://reviews.llvm.org/D145835