In Go, aliases are supposed to be identical to the type being aliased,
so questions about type compatibility need to always ignore aliases,
except for error messages involving the type name.  This patch
implements that.  This test case for this is
https://golang.org/cl/94995.  This fixes
https://golang.org/issue/23912.  Bootstrapped and ran Go testsuite  on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian