Merging r373216:

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r373216 | probinson | 2019-09-30 08:01:35 -0700 (Mon, 30 Sep 2019) | 7 lines

[SSP] [1/3] Revert "StackProtector: Use PointerMayBeCaptured"
"Captured" and "relevant to Stack Protector" are not the same thing.

This reverts commit f29366b1.
aka r363169.

Differential Revision: https://reviews.llvm.org/D67842
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To avoid changing the ABI, the VisitedPHIs member from the StackProtector class
was replaced with a local variable in StackProtector::RequiresStackProtector().