compiler: permit compiler directives in parenthesized groups

The original compiler directive support was only for //line at the
start of a line and for //go: comments before function declarations.
When support was added for //go:notinheap for types and //go:embed for
variables the code did not adapt to permit spaces before the comment
or to permit the comments in var() or type() groups.  This change
corrects those omissions.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/358699