Move compile_instance to compile.c

Move compile_instance to compile.c

This simple patch moves any code related to compile_instance into
compile.c, reserving compile-c-* files strictly for C language support.

gdb/ChangeLog:
        * compile/compile-c-symbols.c (struct symbol_error)
        (hash_symbol_error, eq_symbol_error, del_symbol_error)
        (compile_instance::insert_symbol_error)
        (compile_instance::error_symbol_once): Move to ...
        * compile/compile.c: ... here.