Introduce la_value_print_inner

Introduce la_value_print_inner

The plan for removing val_print is, essentially, to first duplicate
printing code as needed to use the value API; and then remove the
val_print code.  This makes it possible to do the changes
incrementally while keeping everything working.

This adds a new la_value_print_inner function pointer to struct
language_defn.  Eventually this will replace la_val_print.  This patch
also changes printing to prefer this API, when available -- but no
language defines it yet.

gdb/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* valprint.c (do_val_print): Call la_value_print_inner, if
	available.
	* rust-lang.c (rust_language_defn): Update.
	* p-lang.c (pascal_language_defn): Update.
	* opencl-lang.c (opencl_language_defn): Update.
	* objc-lang.c (objc_language_defn): Update.
	* m2-lang.c (m2_language_defn): Update.
	* language.h (struct language_defn) <la_value_print_inner>: New
	member.
	* language.c (unknown_language_defn, auto_language_defn): Update.
	* go-lang.c (go_language_defn): Update.
	* f-lang.c (f_language_defn): Update.
	* d-lang.c (d_language_defn): Update.
	* c-lang.c (c_language_defn, cplus_language_defn)
	(asm_language_defn, minimal_language_defn): Update.
	* ada-lang.c (ada_language_defn): Update.