Document some safety constraints and use more safe wrappers

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2359,7 +2359,7 @@ unsafe extern "C" {
     );
     pub fn LLVMRustWriteOutputFile<'a>(
         T: &'a TargetMachine,
-        PM: &PassManager<'a>,
+        PM: *mut PassManager<'a>,
         M: &'a Module,
         Output: *const c_char,
         DwoOutput: *const c_char,

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -241,6 +241,10 @@ pub fn set_linkage(llglobal: &Value, linkage: Linkage) {
     }
 }
 
+pub fn is_declaration(llglobal: &Value) -> bool {
+    unsafe { LLVMIsDeclaration(llglobal) == ffi::True }
+}
+
 pub fn get_visibility(llglobal: &Value) -> Visibility {
     unsafe { LLVMGetVisibility(llglobal) }.to_rust()
 }