[NFC][Clang] Use `isa_and_nonnull` instead of `VD && isa<VarDecl>(VD)` (#93207)

This issue was found in https://github.com/llvm/llvm-project/pull/86960

.
But I'd like to avoid mixing together a bunch of cleanups with actual
changes.

Signed-off-by: yronglin <yronglin777@gmail.com>