encoding/json: support TextUnmarshaler for map keys with string underlying types

encoding/json: support TextUnmarshaler for map keys with string underlying types

When unmarshaling to a map, the map's key type must either be a string,
an integer, or implement encoding.TextUnmarshaler. But for a user
defined type, reflect.Kind will not distinguish between the static type
and the underlying type. In:

	var x MyString = "x"
	t := reflect.TypeOf(x)
	println(t.Kind() == reflect.String)

the Kind of x is still reflect.String, even though the static type of x
is MyString.

Moreover, checking for the map's key type is a string occurs first, so
even if the map key type MyString implements encoding.TextUnmarshaler,
it will be ignored.

To fix the bug, check for encoding.TextUnmarshaler first.

Fixes #34437

Change-Id: I780e0b084575e1dddfbb433fe03857adf71d05fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/200237
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>