Convert Rust to use discriminated unions

A Rust enum is, essentially, a discriminated union.  Currently the
Rust language support handles Rust enums locally, in rust-lang.c.
However, because I am changing the Rust compiler to use
DW_TAG_variant* to represent enums, it seemed better to have a single
internal representation for Rust enums in gdb.

This patch implements this idea by moving the current Rust enum
handling code to dwarf2read.  This allows the simplification of some
parts of rust-lang.c as well.

2018-02-26  Tom Tromey  <tom@tromey.com>

	* rust-lang.h (rust_last_path_segment): Declare.
	* rust-lang.c (rust_last_path_segment): Now public.  Change
	contract.
	(struct disr_info): Remove.
	(RUST_ENUM_PREFIX, RUST_ENCODED_ENUM_REAL)
	(RUST_ENCODED_ENUM_HIDDEN, rust_union_is_untagged)
	(rust_get_disr_info, rust_tuple_variant_type_p): Remove.
	(rust_enum_p, rust_enum_variant): New function.
	(rust_underscore_fields): Remove "offset" parameter.
	(rust_print_enum): New function.
	(rust_val_print) <TYPE_CODE_UNION>: Remove enum code.
	<TYPE_CODE_STRUCT>: Call rust_print_enum when appropriate.
	(rust_print_struct_def): Add "for_rust_enum" parameter.  Handle
	enums.
	(rust_internal_print_type): New function, from rust_print_type.
	Remove enum code.
	(rust_print_type): Call rust_internal_print_type.
	(rust_evaluate_subexp) <STRUCTOP_ANONYMOUS, STRUCTOP_STRUCT>:
	Update enum handling.
	* dwarf2read.c (struct dwarf2_cu) <rust_unions>: New field.
	(rust_fully_qualify, alloc_discriminant_info, quirk_rust_enum)
	(rust_union_quirks): New functions.
	(process_full_comp_unit, process_full_type_unit): Call
	rust_union_quirks.
	(process_structure_scope): Update rust_unions if necessary.

2018-02-26  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Accept more possible results in enum test.