[clang] [clang] Fix a crash when a variable is captured by a block nested inside a lambda (PR #93749)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

`hasInit` checks whether `VarDecl::Init`'s pointer is null. But `VarDecl::ensureEvaluatedStmt` creates an `EvaluatedStmt` if there isn't one already,  after which `hasInit` returns true.

https://github.com/llvm/llvm-project/blob/3af717d661e9fe8d562181b933a373ca58e41b27/clang/lib/AST/Decl.cpp#L2541

`Init.get<Stmt *>()` returns null when there's no initializer.

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