cmd/compile/internal/escape: change escapes and persists into bitset

cmd/compile/internal/escape: change escapes and persists into bitset

This CL introduces a locAttr bitset type, which will make it easier to
add additional attributes in the near future.

Change-Id: I2689aa623097279dc1e7b7cf2adf5184d710c5a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/520258
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>