[dev.regabi] cmd/compile: unexport reflectdata.WriteType

WriteType isn't safe for direct concurrent use, and users should
instead use TypeLinksym or another higher-level API provided by
reflectdata. After the previous CL, there are no remaining uses of
WriteType elsewhere in the compiler, so unexport it to keep it that
way.

For #43701.

[git-generate]
cd src/cmd/compile/internal/reflectdata
rf '
	mv WriteType writeType
'

Change-Id: I294a78be570a47feb38a1ad4eaae7723653d5991
Reviewed-on: https://go-review.googlesource.com/c/go/+/284077


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>