Merging r371557:

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r371557 | rsmith | 2019-09-10 14:24:09 -0700 (Tue, 10 Sep 2019) | 7 lines

When evaluating a __builtin_constant_p conditional, always enter
constant-folding mode regardless of the original evaluation mode.

In order for this to be correct, we need to track whether we're checking
for a potential constant expression or checking for undefined behavior
separately from the evaluation mode enum, since we don't want to clobber
those states when entering constant-folding mode.
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