Import the swift-compiler-crashes test suite.

Add all compiler crash tests to the validation-test/compiler_crashers
subdirectory. Add a RUN: line to each test case according to the current
behavior of the Swift compiler.

Swift SVN r23418

validation-test/compiler_crashers/0253-swift-constraints-constraintsystem-constraintsystem.swift

@@ -0,0 +1,11 @@
+// RUN: not %swift %s -parse
+
+// Distributed under the terms of the MIT license
+// Test case submitted to project by https://github.com/practicalswift (practicalswift)
+// Test case found by fuzzing
+
+import Foundat= 0) {
+}
+let c = a
+c()
+func b(c) -> <d>d)