stdlib: add a function to squeeze a number in a given range from a hash value

This function mixes the bits in the hash value, which improves Dictionary
performance for keys with bad hashes.

PrecommitBenchmark changes with greater than 7% difference:

``````````Dictionary2`,```1456.00`,```1508.00`,```1502.00`,````624.00`,````607.00`,````592.00`,`864.00`,``145.9%
``````````Dictionary3`,```1379.00`,```1439.00`,```1408.00`,````585.00`,````567.00`,````552.00`,`827.00`,``149.8%
````````````Histogram`,````850.00`,````849.00`,````851.00`,```1053.00`,```1049.00`,```1048.00`,`199.00`,``-19.0%
````````````````Prims`,```1999.00`,```2005.00`,```2018.00`,```1734.00`,```1689.00`,```1701.00`,`310.00`,```18.4%
``````````StrSplitter`,```2365.00`,```2334.00`,```2316.00`,```1979.00`,```1997.00`,```2000.00`,`337.00`,```17.0%
```````````````TwoSum`,```1551.00`,```1568.00`,```1556.00`,```1771.00`,```1741.00`,```1716.00`,`165.00`,```-9.6%

Regressions are in benchmarks that use `Int` as dictionary key: we are just
doing more work than previously (hashing an `Int` was an identity function).

rdar://17962402


Swift SVN r21142

test/stdlib/ReflectionHashing.swift

@@ -0,0 +1,52 @@
+// RUN: %target-build-swift -module-name a %s -o %t.out
+// RUN: %target-run %t.out
+
+//
+// This file contains reflection tests that depend on hash values.
+// Don't add other tests here.
+//
+
+import StdlibUnittest
+
+var Reflection = TestCase("Reflection")
+
+Reflection.test("Dictionary/Empty") {
+  let dict = [Int : Int]()
+
+  var output = ""
+  dump(dict, &output)
+
+  var expected = "- 0 key/value pairs\n"
+
+  expectEqual(expected, output)
+}
+
+Reflection.test("Dictionary") {
+  let dict = [ "One": 1, "Two": 2, "Three": 3, "Four": 4, "Five": 5 ]
+
+  var output = ""
+  dump(dict, &output)
+
+  var expected = ""
+  expected += "▿ 5 key/value pairs\n"
+  expected += "  ▿ [0]: (2 elements)\n"
+  expected += "    - .0: Five\n"
+  expected += "    - .1: 5\n"
+  expected += "  ▿ [1]: (2 elements)\n"
+  expected += "    - .0: Two\n"
+  expected += "    - .1: 2\n"
+  expected += "  ▿ [2]: (2 elements)\n"
+  expected += "    - .0: One\n"
+  expected += "    - .1: 1\n"
+  expected += "  ▿ [3]: (2 elements)\n"
+  expected += "    - .0: Three\n"
+  expected += "    - .1: 3\n"
+  expected += "  ▿ [4]: (2 elements)\n"
+  expected += "    - .0: Four\n"
+  expected += "    - .1: 4\n"
+
+  expectEqual(expected, output)
+}
+
+runAllTests()
+