[Issue 17985] Implement -stdin for rdmd

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17985

--- Comment #4 from Andre <andre at s-e-a-p.de> ---
It's great dmd -run supports now -stdin. Thanks a lot. That is viable
workaround.

I would like to keep this issue open to get the same functionality for rdmd,
because:

- rdmd is the advertised tool to work with D in a script like way
- dmd -run is a less knower argument. Most developers will try `cat foo.d |
rdmd -`. Another benefit is, I always write the run argument wrong "--run". Be
able to just write `...| rdmd -` is convenient

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