Adding command line flag for setAssumeStaticInheritanceRequired

[rorye]

Fix for #3559

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[rorye]

@googlebot I signed it!

[brad4d]

I've imported this PR and sent it for internal review and testing.

[brad4d]

FYI, I've had to reverse the name of the flag and the logic around it to be consistent with our flag conventions.
--assume_something=true is expected to always enable more optimizations, not disable optimizations.

I expect to land the modified PR in the next day or two.