reflect: sort exported methods first

By moving exported methods to the front of method lists, filtering
down to only the exported methods just needs a count of how many
exported methods exist, which the compiler can statically
provide. This allows getting rid of the exported method cache.

For #22075.

Change-Id: I8eeb274563a2940e1347c34d673f843ae2569064
Reviewed-on: https://go-review.googlesource.com/100846
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/type.go

@@ -329,7 +329,7 @@ type method struct {
 type uncommontype struct {
 	pkgpath nameOff
 	mcount  uint16 // number of methods
-	_       uint16 // unused
+	xcount  uint16 // number of exported methods
 	moff    uint32 // offset from this uncommontype to [mcount]method
 	_       uint32 // unused
 }