Change parameters to rust_subscript

Change parameters to rust_subscript

This changes the parameters to rust_subscript, making it more suitable
for reuse by the (coming) new expression code.  In particular,
rust_subscript no longer evaluates its subexpressions.  Instead, they
are passed in.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_subscript): Change parameters.
	(rust_evaluate_subexp): Update.