[perl #114222] Make ‘use’ parse arguments in term context

[perl #114222] Make ‘use’ parse arguments in term context

(lexing context, that is)

use constant { () }

was a syntax error, because the lexer was guessing when { should be
a statement or hash.

It should not be doing that where a term is expected.

It was actually getting itself confused, and trying to parse the
argument list as a statement.

Setting PL_expect after force_next is ineffectual, as force_next
records the current value of PL_expect, arranging to have it
restored.

OPERATOR(USE) was setting PL_expect, but too late.  So no we set
PL_expect explicitly in S_tokenize_use, before any forced tokens,
and use TOKEN(USE), which does not set PL_expect (as setting it
there has no effect).