Fix handling of discriminantless univariant enums in Rust; fix bug with encoded enums

Fix handling of discriminantless univariant enums in Rust; fix bug with encoded enums

2016-10-27  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-lang.c (rust_get_disr_info): Treat univariant enums
    without discriminants as encoded enums with a real field
    * rust-lang.c (rust_evaluate_subexp): Handle field access
    on encoded struct-like enums

gdb/testsuite/ChangeLog:
    * simple.rs: Add test for univariant enums without discriminants
    and for encoded struct-like enums
    * simple.exp: Add test expectations