[cdac] Lazily read global string/object method table pointers

[elinor-fung]

When the [c]DAC is created, the string and object method tables may not be loaded and the global pointers set yet. We don't actually need them at creation of SOSDacImpl, so but we were always reading those global pointers and expecting them to be set. Switch reading of the string and object method table pointers to be lazy,

cc @AaronRobinsonMSFT @davidwrighton

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[AaronRobinsonMSFT]

Aside from a minor comment nit, looks good to me.