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/swift-ast-script.cpp

@@ -0,0 +1,93 @@
+//===--- swift-ast-script.cpp ---------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// This utility is a command line tool that searches Swift code for
+/// declarations matching the given requirements.
+///
+//===----------------------------------------------------------------------===//
+
+#include "swift/Frontend/Frontend.h"
+#include "swift/FrontendTool/FrontendTool.h"
+#include "swift/Basic/LLVMInitialize.h"
+#include "ASTScript.h"
+#include "ASTScriptConfiguration.h"
+
+using namespace swift;
+using namespace scripting;
+
+namespace {
+
+class Observer : public FrontendObserver {
+  ArrayRef<const char *> Args;
+  std::unique_ptr<ASTScriptConfiguration> Config;
+  std::unique_ptr<ASTScript> Script;
+  bool HadError = false;
+
+public:
+  Observer(ArrayRef<const char *> args) : Args(args) {}
+
+  void configuredCompiler(CompilerInstance &instance) override {
+    Config = ASTScriptConfiguration::parse(instance, Args);
+    if (!Config) return flagError();
+
+    Script = ASTScript::parse(*Config);
+    if (!Script) return flagError();
+  }
+
+  void performedSemanticAnalysis(CompilerInstance &instance) override {
+    if (Script) {
+      if (Script->execute())
+        return flagError();
+    }
+  }
+
+  bool hadError() const {
+    return HadError;
+  }
+
+private:
+  void flagError() {
+    HadError = true;
+  }
+};
+
+}
+
+int main(int argc, const char *argv[]) {
+  PROGRAM_START(argc, argv);
+
+  // Look for the first "--" in the arguments.
+  auto argBegin = argv + 1;
+  auto argEnd = argv + argc;
+  auto dashDash = std::find(argBegin, argEnd, StringRef("--"));
+  if (dashDash == argEnd) {
+    llvm::errs() << "error: missing '--' in arguments to separate "
+                    "script configuration from compiler arguments\n"
+                    "usage:\n"
+                    "  swift-grep <script_file> -- <compiler flags>\n";
+    return 1;
+  }
+
+  Observer observer(llvm::makeArrayRef(argBegin, dashDash));
+
+  // Set up the frontend arguments.
+  unsigned numFrontendArgs = argEnd - (dashDash + 1);
+  SmallVector<const char *, 8> frontendArgs;
+  frontendArgs.reserve(numFrontendArgs + 1);
+  frontendArgs.append(dashDash + 1, argEnd);
+  frontendArgs.push_back("-typecheck");
+
+  int frontendResult =
+    performFrontend(frontendArgs, argv[0], (void*) &main, &observer);
+
+  return (observer.hadError() ? 1 : frontendResult);
+}