compiler: read embedcfg files, parse go:embed directives

This change reads go:embed directives and attaches them to variables.
We still don't do anything with the directives.

This change also reads the file passed in the -fgo-embedcfg option.

Change-Id: I405e02682500aec12ea6c99423a9bf13c36f9f6a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/281533
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
8 files changed