patch 8.1.1915: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_match.vim b/src/testdir/test_match.vim
index db87914..0bc1863 100644
--- a/src/testdir/test_match.vim
+++ b/src/testdir/test_match.vim
@@ -215,7 +215,7 @@
         \]
   call assert_equal(expect, savematches)
 
-  call clearmatches(winid)
+  eval winid->clearmatches()
   call assert_equal([], getmatches(winid))
 
   call setmatches(savematches, winid)