gdb: move frame_info_ptr to frame.{c,h}

A patch later in this series will make frame_info_ptr access some
fields internal to frame_info, which we don't want to expose outside of
frame.c.  Move the frame_info_ptr class to frame.h, and the definitions
to frame.c.  Remove frame-info.c and frame-info.h.

Change-Id: Ic5949759e6262ea0da6123858702d48fe5673fea
Reviewed-By: Bruno Larsen <blarsen@redhat.com>

gdb/c-lang.h

@@ -25,6 +25,7 @@ struct ui_file;
 struct language_arch_info;
 struct type_print_options;
 struct parser_state;
+struct compile_instance;
 
 #include "compile/compile.h"
 #include "value.h"