Turn remaining value_contents functions into methods

This turns the remaining value_contents functions -- value_contents,
value_contents_all, value_contents_for_printing, and
value_contents_for_printing_const -- into methods of value.  It also
converts the static functions require_not_optimized_out and
require_available to be private methods.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/compile/compile-object-load.c

@@ -585,7 +585,7 @@ store_regs (struct type *regs_type, CORE_ADDR regs_base)
 
       inferior_addr = regs_base + reg_offset;
       if (0 != target_write_memory (inferior_addr,
-				    value_contents (regval).data (),
+				    regval->contents ().data (),
 				    reg_size))
 	error (_("Cannot write register \"%s\" to inferior memory at %s."),
 	       reg_name, paddress (gdbarch, inferior_addr));