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/sys/hermit/fs.rs

@@ -335,6 +335,7 @@ impl File {
         false
     }
 
+    #[inline]
     pub fn flush(&self) -> io::Result<()> {
         Ok(())
     }

library/std/src/sys/unix/fs.rs

@@ -1204,6 +1204,7 @@ impl File {
         self.0.write_vectored_at(bufs, offset)
     }
 
+    #[inline]
     pub fn flush(&self) -> io::Result<()> {
         Ok(())
     }

library/std/src/sys/unix/stdio.rs

@@ -54,6 +54,7 @@ impl io::Write for Stdout {
         true
     }
 
+    #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }
@@ -81,6 +82,7 @@ impl io::Write for Stderr {
         true
     }
 
+    #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }