Lexer: pass updated buffer to actions (#22201)

In the lexer, predicates have the following type:
	{ ... } :: user       -- predicate state
		-> AlexInput  -- input stream before the token
		-> Int        -- length of the token
		-> AlexInput  -- input stream after the token
		-> Bool       -- True <=> accept the token
This is documented in the Alex manual.

There is access to the input stream both before and after the token.
But when the time comes to construct the token, GHC passes only the
initial string buffer to the lexer action. This patch fixes it:

	- type Action = PsSpan -> StringBuffer -> Int ->                 P (PsLocated Token)
	+ type Action = PsSpan -> StringBuffer -> Int -> StringBuffer -> P (PsLocated Token)

Now lexer actions have access to the string buffer both before and after
the token, just like the predicates. It's just a matter of passing an
additional function parameter throughout the lexer.