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/ASTScriptParser.cpp

@@ -0,0 +1,118 @@
+//===--- ASTScriptParser.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
+//
+//===----------------------------------------------------------------------===//
+///
+/// AST script parsing.
+///
+//===----------------------------------------------------------------------===//
+
+#include "swift/Basic/QuotedString.h"
+#include "swift/Frontend/Frontend.h"
+#include "swift/Parse/Lexer.h"
+#include "swift/Parse/Token.h"
+#include "llvm/Support/MemoryBuffer.h"
+
+#include "ASTScript.h"
+#include "ASTScriptConfiguration.h"
+
+using namespace swift;
+using namespace scripting;
+
+namespace {
+
+class ASTScriptParser {
+  ASTScriptConfiguration &Config;
+  ASTContext &Context;
+  DiagnosticEngine &Diags;
+  std::unique_ptr<llvm::MemoryBuffer> Buffer;
+  Optional<Lexer> TheLexer;
+  Token Tok;
+
+public:
+  ASTScriptParser(ASTScriptConfiguration &config)
+    : Config(config), Context(config.Compiler.getASTContext()),
+      Diags(config.Compiler.getDiags()) {
+
+    auto &compiler = config.Compiler;
+
+    // Open the file and give it to the source manager.
+    auto bufferOrErr = llvm::MemoryBuffer::getFile(config.ScriptFile);
+    if (!bufferOrErr) {
+      llvm::errs() << "error opening file " << QuotedString(config.ScriptFile)
+                   << ": " << bufferOrErr.getError().message() << "\n";
+      return;
+    }
+    auto &sourceManager = compiler.getSourceMgr();
+    auto bufferID = sourceManager.addNewSourceBuffer(std::move(*bufferOrErr));
+
+    // Build and prime the lexer.
+    TheLexer.emplace(Context.LangOpts, sourceManager, bufferID,
+                     &Diags, LexerMode::Swift);
+    TheLexer->lex(Tok);
+  }
+
+  std::unique_ptr<ASTScript> parseFile() {
+    if (!TheLexer) return nullptr;
+    return parseTopLevel();
+  }
+
+private:
+  /***************************************************************************/
+  /*** Parsing primitives ****************************************************/
+  /***************************************************************************/
+
+  void consume(tok kind) {
+    assert(Tok.is(kind));
+    TheLexer->lex(Tok);
+  }
+
+  bool consumeIf(tok kind) {
+    if (Tok.isNot(kind)) return false;
+    consume(kind);
+    return true;
+  }
+
+  bool consumeIfExactly(StringRef literal) {
+    if (Tok.isNot(tok::identifier) || Tok.getText() != literal)
+      return false;
+    consume(tok::identifier);
+    return true;
+  }
+
+  bool consumeIfIdentifier(StringRef &ident) {
+    if (Tok.isNot(tok::identifier)) return false;
+    ident = Tok.getText();
+    consume(tok::identifier);
+    return true;
+  }
+
+  Optional<StringRef> consumeIfIdentifier() {
+    StringRef ident;
+    return consumeIfIdentifier(ident) ? Optional<StringRef>(ident) : None;
+  }
+
+  /***************************************************************************/
+  /*** ASTScript parsing *****************************************************/
+  /***************************************************************************/
+
+  std::unique_ptr<ASTScript> parseTopLevel();
+};
+
+} // end anonymous namespace
+
+/// ast-script ::= ???
+std::unique_ptr<ASTScript> ASTScriptParser::parseTopLevel() {
+  return std::unique_ptr<ASTScript>(new ASTScript(Config));
+}
+
+std::unique_ptr<ASTScript> ASTScript::parse(ASTScriptConfiguration &config) {
+  return ASTScriptParser(config).parseFile();
+}