Remove per-language op_name functions

Remove per-language op_name functions

enum exp_opcode is created from all the .def files, but then each
language is required to implement its own op_name function to turn an
enum value to a string.  This seemed over-complicated to me, and this
patch removes the per-language functions in favor of simply using the
.def names for all languages.  Note that op_name is only used for
dumping expressions, which is a maintainer/debug feature.
Furthermore, I don't think there was any case where the .def name and
the string name differed.

gdb/ChangeLog
2020-11-30  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_op_name): Remove.
	(exp_descriptor_rust): Update.
	* parser-defs.h (op_name_standard): Don't declare.
	(struct exp_descriptor) <op_name>: Remove.
	* parse.c (exp_descriptor_standard): Update.
	* opencl-lang.c (exp_descriptor_opencl): Update.
	* m2-lang.c (m2_language::exp_descriptor_modula2): Update.
	* f-lang.c (op_name_f): Remove.
	(f_language::exp_descriptor_tab): Update.
	* expression.h (op_name): Update.
	* expprint.c (op_name): Rewrite.
	(op_name_standard): Remove.
	(dump_raw_expression, dump_subexp): Update.
	* c-lang.c (exp_descriptor_c): Update.
	* ax-gdb.c (gen_expr): Update.
	* ada-lang.c (ada_op_name): Remove.
	(ada_exp_descriptor): Update.