Stub out a tool that runs a script over a Swift AST.

The tool is currently hard-coded to find functions in the SwiftUI library that take parameters of type `(...) -> T` where `T: View` but where the parameter isn't annotated with `@ViewBuilder`.  The long-term vision here, of course, is that this reads and interprets a script file, but that's quite a bit more work (especially to generate a million bindings to the AST).  In the meantime, I think having a functional harness that people familiar with the C++ API can easily hack on to make their own tools is still pretty useful.

The harness does try to open a script file and lex the first token of it, because that's exactly as far as I got before deciding to hard-code the query I wanted.  Since this input is otherwise ignored, you can just point the tool at any old `.swift` file (or just an empty file) and it'll be fine.

tools/swift-ast-script/ASTScript.h

@@ -0,0 +1,40 @@
+//===--- ASTScript.h - AST script type --------------------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// The AST for a swift-ast-script script.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SCRIPTING_ASTSCRIPT_H
+#define SWIFT_SCRIPTING_ASTSCRIPT_H
+
+#include "swift/Basic/LLVM.h"
+
+namespace swift {
+namespace scripting {
+class ASTScriptConfiguration;
+
+class ASTScript {
+  ASTScriptConfiguration &Config;
+
+public:
+  ASTScript(ASTScriptConfiguration &config) : Config(config) {}
+
+  static std::unique_ptr<ASTScript> parse(ASTScriptConfiguration &config);
+
+  bool execute() const;
+};
+
+}
+}
+
+#endif