[clang] Use cast_or_null instead of cast (PR #93749)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

Note that the call to `hasInit` at the beginning of function `VarDecl::getInit` returns true even when the variable doesn't have an initializer if `VarDecl::ensureEvaluatedStmt` is called before that.

https://github.com/llvm/llvm-project/pull/93749