Implement complex arithmetic

Implement complex arithmetic

This adds support for complex arithmetic to gdb.  Now something like
"print 23 + 7i" will work.

Addition, subtraction, multiplication, division, and equality testing
are supported binary operations.

Unary +, negation, and complement are supported.  Following GCC, the ~
operator computes the complex conjugate.

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

	PR exp/25299:
	* valarith.c (promotion_type, complex_binop): New functions.
	(scalar_binop): Handle complex numbers.  Use promotion_type.
	(value_pos, value_neg, value_complement): Handle complex numbers.

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

	* gdb.base/complex-parts.exp: Add arithmetic tests.