[stdlib] Rename 'countElements' => 'count'

The name was not only long and unwieldy, but inconsistent with our
conscious decision to avoid the use of "elements" in APIs as mostly
redundant.

Swift SVN r22408

validation-test/stdlib/UnicodeTrie.swift.gyb

@@ -133,7 +133,7 @@ func checkGraphemeClusterSegmentation(
   var actualBoundaries: [Int] = [ 0 ]
   var unicodeScalarCount = 0
   for c in subject {
-    let currentClusterSize = countElements(String(c).unicodeScalars)
+    let currentClusterSize = count(String(c).unicodeScalars)
     unicodeScalarCount += currentClusterSize
     actualBoundaries += [ unicodeScalarCount ]
   }