Add a lit test showing and documenting how to use builtins in -parse-stdlib mode

Author: kubamracek

test/Frontend/implicit-builtins-import.swift

@@ -0,0 +1,21 @@
+// This is just an example and documentation of how -parse-stdlib implicitly imports the Builtin module.
+// Note that a lot of builtins expect some types to be defined in the stdlib, otherwise the builtin will be unavailable.
+// Therefore "-module-name Swift" is significant (otherwise the types won't be find in the module we're building).
+//
+// Example:
+//   %target-swift-emit-ir -parse-stdlib %s
+//   Builtin.unreachable() // <<< error: module 'Builtin' has no member named 'unreachable'
+//
+// The real problem is that the 'unreachable' builtin needs the Never type (that's its return type).
+//
+// Example:
+//   %target-swift-emit-ir -parse-stdlib -module-name Swift %s
+//   enum Never {}
+//   Builtin.unreachable() // works
+
+// RUN: %target-swift-emit-ir -parse-stdlib -module-name Swift %s
+
+enum Never {}
+
+Builtin.unreachable()
+Builtin.int_trap()