compiler: ignore struct field tags for type conversion

    
    Go 1.8 includes a language change (https://golang.org/doc/go1.8#language):
    in an explicit conversion from one struct type to another, any field
    tags are ignored.
    
    This CL implements this language change in the gofrontend.  The tests
    for this are in the gc testsuite, which will be copied into the gccgo
    repository in due course.
    
    Updates golang/go#16085.
    
    Reviewed-on: https://go-review.googlesource.com/43614

From-SVN: r248248