cmd/compile: import/export of alias declarations

This CL completes support for alias declarations in the compiler.

Also:
- increased export format version
- updated various comments

For #16339.
Fixes #17487.

Change-Id: Ic6945fc44c0041771eaf9dcfe973f601d14de069
Reviewed-on: https://go-review.googlesource.com/32090
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/go.go

@@ -63,6 +63,7 @@ const (
 	SymSiggen
 	SymAsm
 	SymAlgGen
+	SymAlias // alias, original is Sym.Def.Sym
 )
 
 // The Class of a variable/function describes the "storage class"