[CIR] Make cir.alloca alignment mandatory (#172663)
Fixed a crash in `CIRToLLVMAllocaOpLowering` where `cir.alloca`
operations without an explicit alignment attribute caused failures.
Modified the ODS definition of `cir.alloca` to use
`ConfinedAttr<I64Attr, [IntMinValue<0>]>`. This ensures the attribute is
always present.
Added a regression test in `clang/test/CIR/Lowering/alloca.cir`.
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Co-authored-by: Sirui Mu <msrlancern@gmail.com>
