d: Implement expression-based contract syntax

d: Implement expression-based contract syntax

Expression-based contract syntax has been added.  Contracts that consist
of a single assertion can now be written more succinctly and multiple
`in` or `out` contracts can be specified for the same function.

Reviewed-on: https://github.com/dlang/dmd/pull/12106

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd e598f69c0.