Auto merge of #122053 - erikdesjardins:alloca, r=nikic

Stop using LLVM struct types for alloca

The alloca type has no semantic meaning, only the size (and alignment, but we specify it explicitly) matter. Using `[N x i8]` is a more direct way to specify that we want `N` bytes, and avoids relying on LLVM's struct layout. It is likely that a future LLVM version will change to an untyped alloca representation.

Split out from #121577.

r? `@ghost`

compiler/rustc_codegen_llvm/src/abi.rs

@@ -227,7 +227,7 @@ impl<'ll, 'tcx> ArgAbiExt<'ll, 'tcx> for ArgAbi<'tcx, Ty<'tcx>> {
                 //   when passed by value, making it larger.
                 let copy_bytes = cmp::min(scratch_size.bytes(), self.layout.size.bytes());
                 // Allocate some scratch space...
-                let llscratch = bx.alloca(cast.llvm_type(bx), scratch_align);
+                let llscratch = bx.alloca(scratch_size, scratch_align);
                 bx.lifetime_start(llscratch, scratch_size);
                 // ...store the value...
                 bx.store(val, llscratch, scratch_align);