Fix unary + in Ada

Fix unary + in Ada

My previous Ada patches introduced a bug that I found after checkin.
I had incorrectly implemented unary +.  There was a test for the
overloaded case, but no test for the ordinary case.

This patch adds the tests and fixes the bug.
Tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-03-15  Tom Tromey  <tromey@adacore.com>

	* ada-exp.y (simple_exp): Always push a result for unary '+'.

gdb/testsuite/ChangeLog
2021-03-15  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/fixed_points.exp: Add tests of unary + and -.