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).

compiler/rustc_codegen_llvm/src/context.rs

@@ -385,11 +385,16 @@ impl MiscMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                 } else {
                     "rust_eh_personality"
                 };
+                if let Some(llfn) = self.get_declared_value(name) {
+                    llfn
+                } else {
                     let fty = self.type_variadic_func(&[], self.type_i32());
-                self.declare_cfn(name, llvm::UnnamedAddr::Global, fty)
+                    let llfn = self.declare_cfn(name, llvm::UnnamedAddr::Global, fty);
+                    attributes::apply_target_cpu_attr(self, llfn);
+                    llfn
+                }
             }
         };
-        attributes::apply_target_cpu_attr(self, llfn);
         self.eh_personality.set(Some(llfn));
         llfn
     }

src/test/ui/panic-runtime/incompatible-type.rs

@@ -0,0 +1,24 @@
+// Check that rust_eh_personality can have a different type signature than the
+// one hardcoded in the compiler.  Regression test for #70117. Used to fail with:
+//
+// Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
+//
+// build-pass
+// compile-flags: --crate-type=lib -Ccodegen-units=1
+#![no_std]
+#![panic_runtime]
+#![feature(panic_runtime)]
+#![feature(rustc_attrs)]
+
+pub struct DropMe;
+
+impl Drop for DropMe {
+    fn drop(&mut self) {}
+}
+
+pub fn test(_: DropMe) {
+    unreachable!();
+}
+
+#[rustc_std_internal_symbol]
+pub unsafe extern "C" fn rust_eh_personality() {}