Rearrange all the SectionKinds and structure them into a hierarchical

group instead of a bunch of random unrelated ideas.  Provide predicates
to categorize a SectionKind into a group, and use them instead of 
getKind() throughout the code.

This also renames a ton of SectionKinds to be more consistent and
evocative, and adds a huge number of comments on the enums so that
I will hopefully be able to remember how this stuff works long from
now.

llvm-svn: 77129