Auto merge of #86950 - tmiasko:personality, r=nagisa

Use existing declaration of rust_eh_personality

If crate declares `rust_eh_personality`, re-use existing declaration
as otherwise attempts to set function attributes that follow the
declaration will fail (unless it happens to have exactly the same
type signature as the one predefined in the compiler).

Fixes #70117.
Fixes https://github.com/rust-lang/rust/pull/81469#issuecomment-809428126; probably.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1165,7 +1165,7 @@ extern "C" {
     pub fn LLVMBuildLandingPad(
         B: &Builder<'a>,
         Ty: &'a Type,
-        PersFn: &'a Value,
+        PersFn: Option<&'a Value>,
         NumClauses: c_uint,
         Name: *const c_char,
     ) -> &'a Value;