cmd/gc: annotate local variables with unique ids for inlining

cmd/gc: annotate local variables with unique ids for inlining

Avoids problems with local declarations shadowing other names.
We write a more explicit form than the incoming program, so there
may be additional type annotations. For example:

        int := "hello"
        j := 2

would normally turn into

        var int string = "hello"
        var j int = 2

but the int variable shadows the int type in the second line.

This CL marks all local variables with a per-function sequence number,
so that this would instead be:

        var int·1 string = "hello"
        var j·2 int = 2

Fixes #4326.

R=ken2
CC=golang-dev
https://golang.org/cl/6816100