rust/25535 Apply embedded offset to enum variant calculation

Hopefully straightforward (and I didn't miss anything ...).

gdb/ChangeLog
2020-02-19  Doug Evans  <dje@google.com>

	PR rust/25535
	* rust-lang.c (rust_print_enum): Apply embedded_offset to
	rust_enum_variant calculation.

gdb/testsuite/ChangeLog
2020-02-19  Doug Evans  <dje@google.com>

	PR rust/25535
	* gdb.rust/simple.exp: Add test.
	* gdb.rust/simple.rs: Add test.