In go.dev/cl/419674 I added a mechanism to the inliner to allow
inlining to fail gracefully when a function body is missing, but I
missed we already have a mechanism for that: typecheck.HaveInlineBody.

This CL makes it overridable so that unified IR can plug in its
appropriate logic, like it does with the logic for building the
ir.InlinedCallExpr node.

While here, rename inline.NewInline to inline.InlineCall, because the
name "NewInline" is now a misnomer since we initialize it to oldInline
(now named oldInlineCall).

Change-Id: I4e65618d3725919f69e6f43cf409699d20fb797c
Reviewed-on: https://go-review.googlesource.com/c/go/+/427234


TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>