Rewrite LLVM's archive writer in Rust

This allows it to be used by other codegen backends

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1967,3 +1967,7 @@ extern "C" int32_t LLVMRustGetElementTypeArgIndex(LLVMValueRef CallSite) {
 #endif
     return -1;
 }
+
+extern "C" bool LLVMRustIsBitcode(char *ptr, size_t len) {
+  return identify_magic(StringRef(ptr, len)) == file_magic::bitcode;
+}