Fix Ada assignment resolution

Fix Ada assignment resolution

The expression rewrite missed an Ada resolution case.  GDB previously
knew how to disambiguate the right hand side of an assignment, but now
it does not.

This patch fixes the problem and adds the missing test case.

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

	* ada-exp.y (exp1): Handle resolution of the right hand side of an
	assignment.

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

	* gdb.ada/enums_overload/enums_overload_main.adb: New file.
	* gdb.ada/enums_overload/enums_overload.ads: New file.
	* gdb.ada/enums_overload/enums_overload.adb: New file.
	* gdb.ada/enums_overload.exp: New file.