[clang-tidy] Fix false positive on ArrayInitIndexExpr inside ProBoundsConstantArrayIndexCheck

Sometimes in the AST we can have an ArraySubscriptExpr,
where the index is an ArrayInitIndexExpr.
ArrayInitIndexExpr is not a constant, so
ProBoundsConstantArrayIndexCheck reports a warning when
it sees such expression. This expression can only be
implicitly generated, and always appears inside an
ArrayInitLoopExpr, so we shouldn't report a warning.

Differential Revision: https://reviews.llvm.org/D132654