cmd/gc: generate simpler names for closures

cmd/gc: generate simpler names for closures

Fixes #8291

There were several complaints about closure names in the issue tracker.
The first problem is that you see names like net/http.func·001
in profiles, traces, etc. And there is no way to figure out what
is that function.
Another issue is non-US-ascii symbols. All programs out there
should accept UTF-8. But unfortunately it is not true in reality.
For example, less does not render middle dot properly.

This change prepends outer function name to closure name and
replaces middle dot with dot. Now names look like:

main.glob.func1
main.glob.func2
main.glob.func2.1
main.init.1
main.init.1.func1
main.init.1.func1.1
main.main.func1
main.main.func1.1

Change-Id: I725726af88f2ad3ced2e3450f0f06bf459fd91c0
Reviewed-on: https://go-review.googlesource.com/3964
Reviewed-by: Russ Cox <rsc@golang.org>