More incremental refactoring in plpgsql: get rid of gram.y dependencies on

More incremental refactoring in plpgsql: get rid of gram.y dependencies on
yytext.  This is a necessary change if we're going to have a lexer interface
layer that does lookahead, since yytext won't necessarily be in step with
what the grammar thinks is the current token.  yylval and yylloc should
be the only side-variables that we need to manage when doing lookahead.