[TBAA] Tests for invalid tbaa.struct metadata

[juliannagele]

These tests show invalid tbaa.struct metadata that is currently accepted in preparation for a change to the IR Verifier that will then reject it.

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[juliannagele]

I don't think this got labelled, so adding (hopefully the right) people manually: @jdoerfert and @dobbelaj-snps

[dobbelaj-snps]

lgtm

[fhahn]

LGTM, thanks!

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