Add -Xsource flag (front-end behaves like specified version where possible)

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As described in an email by Jason:

I've been plugging a few soundness holes recently, and thought it might be good to make some of the fixes opt-out with a compiler flag.

For example: https://github.com/scala/scala/pull/3178#issuecomment-29444519

We've discussed having a \-source: option in the context of using the 2.11 presentation for 2.10 projects. I could put these sort of things behind that flag rather than inventing a plethora of new \-Ybug\-compat\-SI\-1234 flags.

The current (among others, IDE team) goal is to not only implement that flag but use it to constrain all 2.11 fixes that prevent the type checker from running on a 2.10 code base.

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Imported From: https://issues.scala-lang.org/browse/SI-8126?orig=1
Reporter: @huitseeker
Affected Versions: 2.11.0-M7
Blocks #8402

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@huitseeker said (edited on Jan 17, 2014 10:04:00 AM UTC):
PR 3340 Merge commit b296396

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@adriaanm said (edited on Mar 5, 2014 7:18:14 PM UTC):
I'd like to consolidate this with -Xfuture, which corresponds to running 2.11 under -Xsource:2.12.

See #8361.