[ptr-traits] Merge the MetadataTracking helpers into the Metadata

header.

This is part of a series of patches to allow LLVM to check for complete
pointee types when computing its pointer traits. This is absolutely
necessary to get correct (or reproducible) results for things like how
many low bits are guaranteed to be zero.

The MetadataTracking helpers aren't actually independent. They rely on
constructing a PointerUnion between Metadata and MetadataAsValue
pointers, which requires know the alignment of pointers to those types
which requires them to be complete.

The .cpp file even defined a method declared in Metadata.h! These really
don't seem like something that is separable, and there is no real
layering problem with just placing them together.

llvm-svn: 256531