[CodeComplete] Fix completion at the end of keywords

Summary:
Make completion behave consistently no matter if it is run at the
start, in the middle or at the end of an identifier that happens to
be a keyword or a macro name. Since completion is often ran on
incomplete identifiers, they may turn into keywords by accident.

For example, we should produce same results for all of these
completion points:

    // ^ is completion point.
    ^class
    cla^ss
    class^

Previously clang produced different results for the last case (as if
the completion point was after a space: `class ^`).

This change also updates some offsets in tests that (unintentionally?)
relied on the old behavior.

Reviewers: sammccall, bkramer, arphaman, aaron.ballman

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45887

llvm-svn: 330717