Introduce functionality for defining region ancestor relation

    Add member functions for Regions to query if another Region is a ancestor.  The
    implementation is naive and traverses all parent regions performing one-to-one
    comparisons.  As a side effect, this introduces `getContainingRegion` function
    for Operations and Values to return the Region in which they are defined, and
    for Regions to return the "parent" region if any.

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PiperOrigin-RevId: 245057980