cmd/compile: use int32 for line numbers consistently

- removed lots of unnecessary int(x) casts
- removed parserline() - was inconsistently used anyway
- minor simplifications in dcl.go

Change-Id: Ibf7de679eea528a31c9692ef1c76a1d9b3239211
Reviewed-on: https://go-review.googlesource.com/20131
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -558,7 +558,7 @@ func importtype(pt *Type, t *Type) {
 		copytype(pt.Nod, t)
 		pt.Nod = n // unzero nod
 		pt.Sym.Importdef = importpkg
-		pt.Sym.Lastlineno = int32(parserline())
+		pt.Sym.Lastlineno = lineno
 		declare(n, PEXTERN)
 		checkwidth(pt)
 	} else if !Eqtype(pt.Orig, t) {