Support diagnostic formatting of keyword tokens

Implemented with a new getKeywordSpelling() accessor. Unlike getTokenName() the
result of this function is stable and may be used in diagnostic output.

Uses of this feature are split out into the subsequent commit.
Don't use magic constants in the digraph diagnostic
Simplify diagnostic tag type using the token kind formatter

As far as the parser is concerned the tag type is always a keyword.
Correct underlying integer type of enum TokenKind

This matches up the underlying type against the actual storage type 'unsigned
short' and lets us get rid of some casts while we're at it.

Effort is made to keep this building in pre-C++11 but as with other features
Token will be less efficiently packed in in legacy configurations.