Add a cached to Interface_info_expression::do_type() so as to reuse
    previously created interface types. This change reduces gccgo peak heap
    usage when compiling the "fmt" package from around 16mb to around 10mb.
    
    Fixes golang/go#16334
    
    Reviewed-on: https://go-review.googlesource.com/24890

From-SVN: r239095