swift234

[SourceKit] Add tags for attribute names

As a first foray into annotating attribute, add tags around attribute
names. For now, treat any decl-modifiers as keywords. We will also want
to wrap the whole attribute (including any parameters) into tags as
well, but that will require more work in the callback hanlding.

Also factor the attribute printing to handle any special cases early,
which will simplify wrapping attributes in tags, since we can then just
put the whole switch intside the pre/post callbacks.

rdar://problem/24292226