[dev.typeparams] cmd/compile/internal/types2: report correct line number for missing key

Use the Key position of a syntax.KeyValueExpr (not the position of the
":") when reporting an error for a missing key.

(In go/types, the KeyValueExpr position is the start of the expression
not the ":", so there this works as expected.)

Change-Id: I74147d245927847274cf4e53b4f03dbb5110c324
Reviewed-on: https://go-review.googlesource.com/c/go/+/276813


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>