Fixes NVIDIA#5984.

Currently, `UnixTimeExprMeta` postpones part of the initialization of its state
to `tagExprForGpu()`. This leads to situations where an incompletely initialized
`UnixTimeExprMeta` object is exercised at runtime, such as in NVIDIA#5984.

This change moves the initialization to the constructor, leaving `tagExprForGpu()`
to deal only with tagging.

Signed-off-by: MithunR <mythrocks@gmail.com>

This reverts commit 0101417.

This is the wrong solution. It does not protect against a different,
unsupported time-format used in a window function.

…-5984

This change tags the child expressions of a GpuWindowExecMeta, to ensure
that all dependency expressions initialized correctly.

Certain child expressions might not be completely initialized even after
`GpuOverrides#wrapExpr()` has been called. For instance, `UnixTimeExprMeta`
postpones some of its initialization until `tagExprForGpu()` is called.

By declaring `windowExpressions`, `partitionSpec`, `orderSpec`, etc. as
`childExprs` of `GpuWindowExecMeta`, one guarantees that the
`GpuOverrides` infrastructure will tag them for GPU execution.

Signed-off-by: MithunR <mythrocks@gmail.com>