cmd/compile: cleanup closure.go

The main thing is we now eagerly create the ODCLFUNC node for
closures, immediately cross-link them, and assign fields (e.g., Nbody,
Dcl, Parents, Marks) directly on the ODCLFUNC (previously they were
assigned on the OCLOSURE and later moved to the ODCLFUNC).

This allows us to set Curfn to the ODCLFUNC instead of the OCLOSURE,
which makes things more consistent with normal function declarations.
(Notably, this means Cvars now hang off the ODCLFUNC instead of the
OCLOSURE.)

Assignment of xfunc symbol names also now happens before typechecking
their body, which means debugging output now provides a more helpful
name than "<S>".

In golang.org/cl/66810, we changed "x := y" statements to avoid
creating false closure variables for x, but we still create them for
struct literals like "s{f: x}". Update comment in capturevars
accordingly.

More opportunity for cleanups still, but this makes some substantial
progress, IMO.

Passes toolstash-check.

Change-Id: I65a4efc91886e3dcd1000561348af88297775cd7
Reviewed-on: https://go-review.googlesource.com/100197
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/syntax.go

@@ -471,8 +471,11 @@ type Func struct {
 	// Marks records scope boundary changes.
 	Marks []Mark
 
-	Closgen    int
-	Outerfunc  *Node // outer function (for closure)
+	// Closgen tracks how many closures have been generated within
+	// this function. Used by closurename for creating unique
+	// function names.
+	Closgen int
+
 	FieldTrack map[*types.Sym]struct{}
 	DebugInfo  *ssa.FuncDebug
 	Ntype      *Node // signature