Support parsing Carbon files from strings. (#954)

Also, pass file names as string_view for greater convenience.

executable_semantics/syntax/parse_test.cpp

@@ -0,0 +1,33 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "executable_semantics/syntax/parse.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <string>
+#include <variant>
+
+#include "executable_semantics/common/arena.h"
+
+namespace Carbon {
+namespace {
+
+static constexpr std::string_view FileContents = R"(
+package ExecutableSemanticsTest api;
+
+fn Foo() {}
+)";
+
+TEST(ParseTest, ParseFromString) {
+  Arena arena;
+  std::variant<AST, SyntaxErrorCode> parse_result =
+      ParseFromString(&arena, "file.carbon", FileContents, /*trace=*/false);
+  ASSERT_TRUE(std::holds_alternative<AST>(parse_result));
+  EXPECT_EQ(std::get<AST>(parse_result).declarations.size(), 1);
+}
+
+}  // namespace
+}  // namespace Carbon