restructure code a bit: there are two potential issues

worth asserting about in this code: 1) if the source range
is bogus (begin loc after end loc), and 2) if the client
is trying to highlight a range that is purely whitespace.

It is possible to just silently ignore #2, but it seems like
it is always a bug, so lets keep asserting on this condition,
but with a better assert message.

llvm-svn: 95998