Speed up Vec::clear().

Currently it just calls `truncate(0)`. `truncate()` is (a) not marked as
`#[inline]`, and (b) more general than needed for `clear()`.

This commit changes `clear()` to do the work itself. This modest change
was first proposed in rust-lang#74172, where the reviewer rejected it because
there was insufficient evidence that `Vec::clear()`'s performance
mattered enough to justify the change. Recent changes within rustc have
made `Vec::clear()` hot within `macro_parser.rs`, so the change is now
clearly worthwhile.

Note that this will also benefit `String::clear()`, because it just
calls `Vec::clear()`.

library/alloc/src/vec/mod.rs

@@ -1881,7 +1881,18 @@ impl<T, A: Allocator> Vec<T, A> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn clear(&mut self) {
-        self.truncate(0)
+        let elems: *mut [T] = self.as_mut_slice();
+
+        // SAFETY:
+        // - `elems` comes directly from `as_mut_slice` and is therefore valid.
+        // - Setting `self.len` before calling `drop_in_place` means that,
+        //   if an element's `Drop` impl panics, the vector's `Drop` impl will
+        //   do nothing (leaking the rest of the elements) instead of dropping
+        //   some twice.
+        unsafe {
+            self.len = 0;
+            ptr::drop_in_place(elems);
+        }
     }
 
     /// Returns the number of elements in the vector, also referred to