Make Rust error message mention the field name

Make Rust error message mention the field name

I noticed a spot in rust-lang.c where the placeholder "foo" was used
instead of the actual field name.  This patch fixes the bug.

gdb/ChangeLog
2018-09-13  Tom Tromey  <tom@tromey.com>

	PR rust/23650:
	* rust-lang.c (rust_evaluate_subexp): Use field name, not "foo".

gdb/testsuite/ChangeLog
2018-09-13  Tom Tromey  <tom@tromey.com>

	PR rust/23650:
	* gdb.rust/simple.exp: Add test for enum field access error.