runtime: rename inlinedCall.func_ to inlinedCall.nameOff

runtime: rename inlinedCall.func_ to inlinedCall.nameOff

The meaning of this field is unchanged, this CL simply gives it a more
descriptive name, as func_ makes it sound like a reference to the _func.

Change-Id: I70e54f34bede7636ce4d7b9dd0f7557308f02143
Reviewed-on: https://go-review.googlesource.com/c/go/+/427961
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>