[release-branch.go1.11] cmd/compile: don't crash reporting misuse of shadowed built-in function

The existing implementation causes a compiler panic if a function parameter shadows a built-in function, and then calling that shadowed name.

Updates #27356
Fixes #27399

Change-Id: I1ffb6dc01e63c7f499e5f6f75f77ce2318f35bcd
Reviewed-on: https://go-review.googlesource.com/132876


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 4a095b87)
Reviewed-on: https://go-review.googlesource.com/c/139103


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>