Add accessors for members of complex numbers

Add accessors for members of complex numbers

This introduces two new functions that make it simpler to access the
components of a complex number.

gdb/ChangeLog
2020-04-01  Tom Tromey  <tom@tromey.com>

	* valprint.c (generic_value_print_complex): Use accessors.
	* value.h (value_real_part, value_imaginary_part): Declare.
	* valops.c (value_real_part, value_imaginary_part): New
	functions.
	* value.c (creal_internal_fn, cimag_internal_fn): Use accessors.