This patch adds an explicit conversion between type aliases.
Otherwise we can get a crash in the backend when GCC sees a
fold_convert_loc between two record types that the GCC backend appear
to be different.  Adding the explicit conversion will insert a
VIEW_CONVERT_EXPR where needed.  The test case is
https://golang.org/cl/73790.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian