When evaluating a __builtin_constant_p conditional, always enter

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@371557 91177308-0d34-0410-b5e6-96231b3b80d8