[NFC] Split up PassBuilder.cpp

PassBuilder.cpp is the slowest file to compile in LLVM.
When trying to test changes to pipelines, it takes a long time to recompile.

This doesn't actually speedup building PassBuilder.cpp itself since most
of the time is spent in other large/duplicated functions caused by
PassRegistry.def.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D109798