[binutils-gdb/gdb-8.2-branch] Make Rust error message mention the field name

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT fde24e1a534b7df3edd0fc167d0ace7d240021f8 ***

Author: Tom Tromey <tom@tromey.com>
Branch: gdb-8.2-branch
Commit: fde24e1a534b7df3edd0fc167d0ace7d240021f8

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.

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.