[NFC] [DwarfEHPrepare] Add additional stats for EH

Stats added:

1. NumCleanupLandingPadsUnreachable: how many cleanup landing pads were optimized as unreachable
1. NumCleanupLandingPadsRemaining: how many cleanup landing pads remain
1. NumNoUnwind: Number of functions with nounwind attribute
1. NumUnwind: Number of functions with unwind attribute

DwarfEHPrepare is always run a single time as part of `TargetPassConfig::addISelPasses()` which makes it an ideal place near the end of the pipeline to record this information.

Example output from clang built with exceptions cumulative during thinLTO backend (NumCleanupLandingPadsUnreachable was not incremented):

	"dwarfehprepare.NumCleanupLandingPadsRemaining": 123660,
	"dwarfehprepare.NumNoUnwind": 323836,
	"dwarfehprepare.NumUnwind": 472893,

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D104161