[clang-format] Parse the else part of `#if 0`

Fixes https://github.com/llvm/llvm-project/issues/57539

Previously things outside of `#if` blocks were parsed as if only the
first branch of the conditional compilation branch existed, unless the
first condition is 0.  In that case the outer parts would be parsed as
if nothing inside the conditional parts existed.  Now we use the second
conditional branch if the first condition is 0.

Reviewed By: owenpan

Differential Revision: https://reviews.llvm.org/D133647