Rollup merge of #114194 - thomcc:flushinline, r=cuviper

Inline trivial (noop) flush calls

At work I noticed that `writer.flush()?` didn't get optimized away in cases where the flush is obviously a no-op, which I had expected (well, desired).

I went through and added `#[inline]` to a bunch of cases that were obviously noops, or delegated to ones that were obviously noops. I omitted platforms I don't have access to (some tier3). I didn't do this very scientifically, in cases where it was non-obvious I left `#[inline]` off.

library/std/src/process.rs

@@ -280,6 +280,7 @@ impl Write for ChildStdin {
         io::Write::is_write_vectored(&&*self)
     }
 
+    #[inline]
     fn flush(&mut self) -> io::Result<()> {
         (&*self).flush()
     }
@@ -299,6 +300,7 @@ impl Write for &ChildStdin {
         self.inner.is_write_vectored()
     }
 
+    #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }