BPF: fix enum value 0 issue for __builtin_preserve_enum_value()

Lorenz Bauer reported that the following code will have
compilation error for bpf target:
    enum e { TWO };
    bpf_core_enum_value_exists(enum e, TWO);
The clang emitted the following error message:
    __builtin_preserve_enum_value argument 1 invalid

In SemaChecking, an expression like "*(enum NAME)1" will have
cast kind CK_IntegralToPointer, but "*(enum NAME)0" will have
cast kind CK_NullToPointer. Current implementation only permits
CK_IntegralToPointer, missing enum value 0 case.

This patch permits CK_NullToPointer cast kind and
the above test case can pass now.

Differential Revision: https://reviews.llvm.org/D97659