Refactor AsmPrinterHandler callbacks. NFCI.

The existing behaviors and callbacks were overlapping and had very
confusing semantics: beginBasicBlock/endBasicBlock were not always
called, beginFragment/endFragment seemed like they were meant to mean
the same thing, but were slightly different, etc. This resulted in
confusing semantics, virtual method overloads, and control flow.

Remove the above, and replace with new beginBasicBlockSection and
endBasicBlockSection callbacks. And document them.

These are always called before the first and after the last blocks in
a function, even when basic-block-sections are disabled.