compiler: improvements for type alias handling

compiler: improvements for type alias handling

    Give an error for an attempt to define a method on an imported type.

    Give an error for each attempt to define a method on a builtin type.

    Adjust error messages to be closer to gc error messages.

    With these changes gccgo passes current tests on dev.typealias branch.

    This changes the errors printed for test/fixedbugs/issue5089.go, but
    the change is an improvement:

        Before:
            fixedbugs/issue5089.go:13:1: error: redefinition of ‘bufio.Buffered’: receiver name changed
             func (b *bufio.Reader) Buffered() int { // ERROR "non-local|redefinition"
             ^
            fixedbugs/issue5089.go:11:13: note: previous definition of ‘bufio.Buffered’ was here
             import "bufio" // GCCGO_ERROR "previous"
                         ^

        Now:
            fixedbugs/issue5089.go:13:7: error: may not define methods on non-local type
             func (b *bufio.Reader) Buffered() int { // ERROR "non-local|redefinition"
                   ^

    Reviewed-on: https://go-review.googlesource.com/35642

From-SVN: r244889