compiler: support go:noescape cross package

compiler: support go:noescape cross package

    CL 83876 added support of go:noescape pragma, but it only works
    for functions called from the same package. The pragma did not
    take effect for exported functions that are not called from
    the same package. The reason is that top level function
    declarations are not traversed, and only reached from calls
    from other functions. This CL adds this support. The Traverse
    class is extended with a mode to traverse function declarations.

    Reviewed-on: https://go-review.googlesource.com/85637

From-SVN: r256405