[ADT] add StringViewExtras llvm::starts_with for std::string_view

std::string_view::starts_with isn't available until C++20. Create
llvm::starts_with for now; we can delete this when LLVM moves to C++20
one day.

To run the newly added unit test:
$ cd llvm/build; ninja ADTTests; cd -
$ ./llvm/build/unittests/ADT/ADTTests --gtest_filter=StringViewExtrasTest.\*

Reviewed By: MaskRay, erichkeane

Differential Revision: https://reviews.llvm.org/D148367

llvm/include/llvm/ADT/StringViewExtras.h

@@ -0,0 +1,35 @@
+//===- llvm/ADT/StringViewExtras.h - Useful string_view functions C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file contains some functions that are useful when dealing with
+/// string_views.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_STRINGVIEWEXTRAS_H
+#define LLVM_ADT_STRINGVIEWEXTRAS_H
+
+#include <string_view>
+
+namespace llvm {
+
+// FIXME: std::string_view::starts_with is not available until C++20. Once LLVM
+// is upgraded to C++20, remove this header and users.
+
+inline constexpr bool starts_with(std::string_view self, char C) {
+  return !self.empty() && self.front() == C;
+}
+inline constexpr bool starts_with(std::string_view haystack,
+                                  std::string_view needle) {
+  return haystack.substr(0, needle.size()) == needle;
+}
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_STRINGVIEWEXTRAS_H

llvm/unittests/ADT/CMakeLists.txt

@@ -79,6 +79,7 @@ add_llvm_unittest(ADTTests
   StringRefTest.cpp
   StringSetTest.cpp
   StringSwitchTest.cpp
+  StringViewExtrasTest.cpp
   TinyPtrVectorTest.cpp
   TwineTest.cpp
   TypeSwitchTest.cpp

llvm/unittests/ADT/StringViewExtrasTest.cpp

@@ -0,0 +1,32 @@
+//===- StringExtrasTest.cpp - Unit tests for String extras ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/StringViewExtras.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+#include <string_view>
+
+using namespace llvm;
+
+TEST(StringViewExtrasTest, starts_with) {
+  std::string haystack = "hello world";
+  EXPECT_TRUE(llvm::starts_with(haystack, 'h'));
+  EXPECT_FALSE(llvm::starts_with(haystack, '\0'));
+  EXPECT_TRUE(llvm::starts_with(haystack, "hello"));
+  // TODO: should this differ from \0?
+  EXPECT_TRUE(llvm::starts_with(haystack, ""));
+
+  std::string empty;
+  EXPECT_FALSE(llvm::starts_with(empty, 'h'));
+  EXPECT_FALSE(llvm::starts_with(empty, '\0'));
+  EXPECT_FALSE(llvm::starts_with(empty, "hello"));
+  // TODO: should this differ from \0?
+  EXPECT_TRUE(llvm::starts_with(empty, ""));
+}