[release-branch.go1.13] internal/reflectlite: update reflectlite to match runtime rtype/mapType

Similar to tip https://golang.org/cl/197559.

Fixes #35408

Change-Id: Ie8e28b93fb3adf23c3a0a39f6917ff76abf44fdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/205721
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: ianlancetaylor

src/internal/reflectlite/type.go

@@ -246,6 +246,7 @@ type mapType struct {
 	rtype
 	key        *rtype // map key type
 	elem       *rtype // map element (value) type
+	bucket     *rtype // internal bucket structure
 	keysize    uint8  // size of key slot
 	valuesize  uint8  // size of value slot
 	bucketsize uint16 // size of bucket