[PATCH] Disallow in-source builds

[Eric Christopher]

Hi Philip:

A stream of comments :)

We've started warning recently, it is best practice, we don't want to worry about breaking it, the bots have all been updated, and I don't know any of the long standing developers who use this that we haven't spoken to - who were you thinking of precisely?

-eric


http://reviews.llvm.org/D10614

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