This patch to the Go frontend permits converting unnamed types when
ignoring struct tags.  I think the code was written this way because
before conversion could ignore struct tags this case could only arise
with named types.  Now that conversions permit struct tags to change,
this can occur with unnamed types as well.  The test case is
https://golang.org/cl/375796.  This fixes
https://golang.org/issue/50439.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian