Auto merge of #116096 - cjgillot:debuginfo-fndef-size, r=nikic

Make FnDef 1-ZST in LLVM debuginfo.

Discussed in https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/LLVM.20HEAD.20llvm.2Edbg.2Edeclare.2Falloca.20size.20mismatch

r? `@nikic`