glsl: Rework reserved word/keyword handling in the lexer.

This consolidates the TOKEN_OR_IDENTIFIER and RESERVED_WORD macros into
a single KEYWORD macro.

The old TOKEN_OR_IDENTIFIER macros handled the case of a word going from
an identifier to a keyword; the RESERVED_WORD macro handled a word going
from a reserved word to a language keyword.  However, neither could
properly handle samplerBuffer (for example), which is an identifier in
1.10 and 1.20, a reserved word in 1.30, and a keyword in 1.40 and on.

Furthermore, the existing macros didn't properly handle reserved words
in GLSL ES 1.00.  The best they could do was return a token (rather than
an identifier), resulting in an obtuse parser error, rather than a
user-friendly "you used a reserved word" error message.

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