Avoid an explicit cast from *const c_uchar to *const c_char

As noted in the `ffi` module docs, passing pointer/length byte strings from
Rust to C++ is easier if we declare them as `*const c_uchar` on the Rust side,
but `const char *` (possibly signed) on the C++ side. This is allowed because
both pointer types are ABI-compatible, regardless of char signedness.

compiler/rustc_codegen_llvm/src/back/owned_target_machine.rs

@@ -1,4 +1,4 @@
-use std::ffi::{CStr, c_char};
+use std::ffi::CStr;
 use std::marker::PhantomData;
 use std::ptr::NonNull;
 
@@ -71,7 +71,7 @@ impl OwnedTargetMachine {
                 output_obj_file.as_ptr(),
                 debug_info_compression.as_ptr(),
                 use_emulated_tls,
-                args_cstr_buff.as_ptr() as *const c_char,
+                args_cstr_buff.as_ptr(),
                 args_cstr_buff.len(),
                 use_wasm_eh,
             )

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2438,7 +2438,7 @@ unsafe extern "C" {
         OutputObjFile: *const c_char,
         DebugInfoCompression: *const c_char,
         UseEmulatedTls: bool,
-        ArgsCstrBuff: *const c_char,
+        ArgsCstrBuff: *const c_uchar, // See "PTR_LEN_STR".
         ArgsCstrBuffLen: usize,
         UseWasmEH: bool,
     ) -> *mut TargetMachine;