Add known type checker crashers.

Add minimized tests for:
  rdar://problem/27464577
  rdar://problem/27575060
  rdar://problem/27680407
  rdar://problem/27787341
  rdar://problem/27815848

As these (and other tests added here) are fixed, we can move them into a
new type_checker_crashers_fixed directory.

Author: rudkx

validation-test/Sema/type_checker_crashers/rdar27464577.swift

@@ -0,0 +1,3 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+_ = [1].map { v -> T in return v as? Int }

validation-test/Sema/type_checker_crashers/rdar27575060.swift

@@ -0,0 +1,7 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+// REQUIRES: asserts
+
+func f(_ x: Any...) {}
+
+var a = 1
+f((a, 2))

validation-test/Sema/type_checker_crashers/rdar27680407.swift

@@ -0,0 +1,10 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+struct rdar27680407 : ExpressibleByStringLiteral {
+  let value: String
+
+  // Stack overflow while validating rdar27680407.StringLiteralType.
+  init(stringLiteral value: rdar27680407.StringLiteralType) {
+    self.value = value
+  }
+}

validation-test/Sema/type_checker_crashers/rdar27787341.swift

@@ -0,0 +1,3 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+_ = [1].reduce([:]) { $0[$1] }

validation-test/Sema/type_checker_crashers/rdar27815848.swift

@@ -0,0 +1,13 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+class C {}
+class D : C {}
+
+@_silgen_name("consume")
+func consume(_: [C])  // note, returns ()
+
+// Assert/crash while emitting diagnostic for coercion from () to Bool
+// in the context of a collection cast.
+func test(x: [D]) -> Bool {
+  return consume(x)  // no way to coerce from () to Bool
+}