`.clang_arg("-x c++")` causes failure in `generate`

As reported by @virtualritz in #1852, using clang_arg("-x c++") causes bindgen's generate to return an Err result.

As a workaround, .clang_arg("-xc++") can be used.

From this build.rs,

use std::path::PathBuf;

fn main() {
    let bindings = bindgen::Builder::default()
        .header("wrapper.hpp")
        .clang_arg("-x c++")            // does not work
        .generate()
        .expect("Couldn't make bindings!");

    let out_path = PathBuf::from(std::env::var("OUT_DIR").unwrap());
    bindings.write_to_file(out_path.join("bindings.rs"))
        .expect("Couldn't write bindings!");
}

RUST_BACKTRACE=1 cargo build -vv shows :

error: language not recognized: ' c++', err: true
warning: '-x c++': 'linker' input unused [-Wunused-command-line-argument], err: false
thread 'main' panicked at 'Couldn't make bindings!: ()', build.rs:4:20
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: std::panicking::rust_panic_with_hook
   6: rust_begin_unwind
   7: core::panicking::panic_fmt
   8: core::result::unwrap_failed
   9: core::result::Result<T,E>::expect
  10: build_script_build::main
  11: std::rt::lang_start::{{closure}}
  12: std::rt::lang_start_internal
  13: std::rt::lang_start
  14: main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Start with the provided build.rs reproduction and inspect the Builder::clang_arg("-x c++") path leading into generate. Run RUST_BACKTRACE=1 cargo build -vv and compare it with the documented -xc++ workaround. Done means the spaced language argument no longer returns an Err while generating bindings.