Remove `inline` keyword from inline `classof` methods

The style guide states that the explicit `inline`
should not be used with inline methods.  classof is
very common inline method with a fair amount on
inconsistency:

$ git grep classof ./include | grep inline | wc -l
230
$ git grep classof ./include | grep -v inline | wc -l
257

I chose to target this method rather the larger change
since this method is easily cargo-culted (I did it at
least once).  I considered doing the larger change and
removing all occurrences but that would be a much larger
change.

Differential Revision: https://reviews.llvm.org/D33906

llvm-svn: 306731