compiler: make sure type descriptor initializers go in .rodata

compiler: make sure type descriptor initializers go in .rodata

    When creating type descriptor variables, the compiler (specifically
    Type::make_type_descriptor_var) invokes the immutable_struct() and
    immutable_struct_set_init() back end methods, so as to insure that
    these items go into the ".rodata" section of the generate object file.
    The expression initializers for these variables can contain nested
    subexpressions, however, and these subexpressions were not always
    being placed into .rodata. This patch changes the backend-gen code for
    slice initializers to emit implicit variables into .rodata if those
    initializers are part of a type descriptor init.

    Reviewed-on: https://go-review.googlesource.com/132596

From-SVN: r264181