Warn about unsupported codegen with the diags machinery, giving us:

t.c:3322:5: warning: cannot codegen this yet
    __asm__ ("bswap   %0" : "+r" (_data));
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

instead of:

Unimplemented stmt!
(AsmStmt 0x80eaa0 <t.c:3331:5, line:3334:28>)

llvm-svn: 44501