cmd/compile: more error position tests for the typechecker

This change adds line position tests for several yyerror calls in the
typechecker that are currently not tested in any way.

Untested yyerror calls were found by replacing them with

  yerrorl(src.NoXPos, ...)

(thus destroying position information in the error), and then running
the test suite. No failures means no test coverage for the relevant
yyerror call.

For #19683

Change-Id: Iedb3d2f02141b332e9bfa76dbf5ae930ad2fddc3
Reviewed-on: https://go-review.googlesource.com/41477
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/interface/explicit.go

@@ -54,6 +54,11 @@ func main() {
 
 	e = E(t) // ok
 	t = T(e) // ERROR "need explicit|need type assertion|incompatible"
+
+	// cannot type-assert non-interfaces
+	f := 2.0
+	_ = f.(int) // ERROR "non-interface type"
+
 }
 
 type M interface {