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/ASTScriptConfiguration.cpp

@@ -0,0 +1,95 @@
+//===--- ASTScriptConfiguration.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
+//
+//===----------------------------------------------------------------------===//
+///
+/// Configuration parsing for an AST script.
+///
+//===----------------------------------------------------------------------===//
+
+#include "ASTScriptConfiguration.h"
+
+#include "swift/Basic/QuotedString.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace swift;
+using namespace scripting;
+
+std::unique_ptr<ASTScriptConfiguration>
+ASTScriptConfiguration::parse(CompilerInstance &compiler,
+                              ArrayRef<const char *> args) {
+  bool hadError = false;
+
+  std::unique_ptr<ASTScriptConfiguration> result(
+    new ASTScriptConfiguration(compiler));
+
+#define emitError(WHAT) do {                                                  \
+    llvm::errs() << "error: " << WHAT << "\n";                                \
+    hadError = true;                                                          \
+  } while (0)
+
+  auto popArg = [&](const char *what) -> StringRef {
+    // Gracefully handle the case of running out of arguments.
+    if (args.empty()) {
+      assert(what && "expected explanation here!");
+      emitError(what);
+      return "";
+    }
+
+    auto arg = args.front();
+    args = args.slice(1);
+    return arg;
+  };
+
+  auto setScriptFile = [&](StringRef filename) {
+    if (!result->ScriptFile.empty()) {
+      emitError("multiple script files ("
+                  << QuotedString(result->ScriptFile)
+                  << ", "
+                  << QuotedString(filename)
+                  << ")");
+    }
+    result->ScriptFile = filename;
+  };
+
+  // Parse the arguments.
+  while (!hadError && !args.empty()) {
+    StringRef arg = popArg(nullptr);
+    if (!arg.startswith("-")) {
+      setScriptFile(arg);
+    } else if (arg == "-f") {
+      StringRef filename = popArg("expected path after -f");
+      if (!hadError)
+        setScriptFile(filename);
+    } else if (arg == "-h" || arg == "--help") {
+      llvm::errs()
+        << "usage: swift-ast-script <script-args> -- <compiler-args\n"
+           "accepted script arguments:\n"
+           "  <filename>\n"
+           "  -f <filename>\n"
+           "    Specify the file to use as the script; required argument\n";
+      hadError = true;
+    } else {
+      emitError("unknown argument " << QuotedString(arg));
+    }
+  }
+
+  // Check well-formedness.
+  if (!hadError) {
+    if (result->ScriptFile.empty()) {
+      emitError("script file is required");
+    }
+  }
+
+  if (hadError)
+    result.reset();
+  return result;
+}