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/1720-swift-clangmoduleunit-getimportedmodules.swift

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t/clang-module-cache
+// RUN: not --crash %swift %s -sdk %sdk -module-cache-path %t/clang-module-cache -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 Foundation
+return m(n: A<T : C(start, let f = ""
+assert(T>(A, ")
+class A<T where d.c : X.c {
+}
+let h: String = A? = A.startInd