[clang] [NFC] Minor fix to tryEmitAbstract type in EmitCXXNewAllocSize (PR #123433)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I ran into this while porting this code to the CIR generator because the constant emitter there was asserting that the destination type was an integer type. My motivation for posting this change here is to make sure my understanding of the expected destination type is correct.

https://github.com/llvm/llvm-project/pull/123433