This commit does not change the semantics of the Call method. Its
purpose is to avoid duplication of code by making PrepareCall available
for separate use by the wasm backend.

Updates #18892

Change-Id: I04a3098f56ebf0d995791c5375dd4c03b6a202a3
Reviewed-on: https://go-review.googlesource.com/103275Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>