[release-branch.go1.1] cmd/gc: fix missing export data for inlining in a few other cases.

««« CL 10464043 / c224c549a3c7
cmd/gc: fix missing export data for inlining in a few other cases.

Exported inlined functions that perform a string conversion
using a non-exported named type may miss it in export data.

Fixes #5755.

R=rsc, golang-dev, ality, r
CC=golang-dev
https://golang.org/cl/10464043
»»»

Update #5928

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11629044