cmd/compile: eliminate base.Pos and ir.CurFunc

This is an umbrella issue for eliminating the global lineno variable from package gc. Filing as a HelpWanted issue because I think it's a relatively low-barrier-to-entry issue that people can usefully contribute to the compiler with small incremental improvements.

Here's the general algorithm:

1. Identify a function Bar that uses lineno.
2. Split it into two functions Bar and Barl; Barl takes an additional pos src.XPos parameter, and Bar just calls Barl with lineno.
3. Change callers of Bar to use Barl. Ideally, the caller can supply the position information directly (e.g., via n.Pos or something); but if necessary, just use lineno and later we'll recursively apply this procedure on the caller function.

An example of this is yyerror and Warn now have yyerrorl and `Warnl' functions that we're trying to use instead.

Note: sometimes uses of lineno should just go away entirely (e.g., CL 38393 / 80c4b53e1e5159cc440e52c906583edc1eb79abc). I suggest pinging here to discuss instances and/or express interest in working on this to avoid duplicating work (e.g., @josharian and I are looking at this in the backend for #15756).