[llvm] [OMPIRBuilder] Replace getAllocatedType with getAllocationSize (PR #181844)

[Jameson Nash via llvm-commits]

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vtjnash wrote:

I was just going to fix the test. Alternatively, I could rebuild a struct here from the components (their types happen to already be available here), I just thought it was a bit cleaner to re-use the existing struct.

https://github.com/llvm/llvm-project/pull/181844