[lldb/Utility] Make StructuredData::Dictionary::GetKeys return an Array

This patch changes `StructuredData::Dictionary::GetKeys` return type
from an `StructuredData::ObjectSP` to a `StructuredData::ArraySP`.

The function already stored the keys in an array but implicitely upcasted
it to an `ObjectSP`, which required the user to convert it again to a
Array object to access each element.

Since we know the keys should be held by an iterable container, it makes
more sense to return the allocated ArraySP as-is.

Differential Revision: https://reviews.llvm.org/D122426



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>