[mlir] Change the order of members in MLIRContext to ensure dialects are destroyed first

Currently, the dialects precede the registered operations in the context object, which means that the latter is destroyed first. At the same time, Operation::~Operation dereferences the registered operation when destroying properties, which can cause use-after-free (e.g. if a dialect owns an op). This patch fixes that by changing the order of the members so that dialects come after registered operations.

Differential Revision: https://reviews.llvm.org/D151440