llvm.is.constant (and therefore Clang's __builtin_constant_p()) need to
report undefined values as non-constant or future DCE choices end up
making no sense. This was encountered while building the Linux kernel
which uses __builtin_constant_p() while trying to evaluate if it is safe
to use a compile-time constant resolution for string lengths or if it
must kick over to a full runtime call to strlen(). Obviously an
undefined variable cannot be known at compile-time, so
__builtin_constant_p() needs to return false. This change will also mean
that Clang will match GCC's behavior under the same conditions.
Fixes#130649
ea2e66aa8b