Factor out thread-safe uniqu'ing backed by vector in MLIRcontext

    Extract common code from getAffineSymbolExpr and getAffineConstantExpr into a
    utility function safeGetOrCreate, similarly to the existing overloads for sets
    and maps.  The position in the vector is used as indexing key.  NFC.

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PiperOrigin-RevId: 244820859