[binutils-gdb] Add support for untagged unions in Rust

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT b96645f1a1e6084ed6a20ddf615bdc4d2487a9c9 ***

Author: Manish Goregaokar <manish@mozilla.com>
Branch: master
Commit: b96645f1a1e6084ed6a20ddf615bdc4d2487a9c9

Add support for untagged unions in Rust

2016-10-28  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-lang.c (rust_union_is_untagged): Add function to
    check if a union is an untagged unioni
    (rust_val_print): Handle printing of untagged union values
    (rust_print_type): Handle printing of untagged union types
    (rust_evaluate_subexp): Handle evaluating field
    access on untagged unions