Replace StringPiece with std::string_view in Signature.

And also in Signature-related code. Remove the function
DexFile::CreateSignature() which was used only in a test
as the test can use method searching functions that take
std::string_view instead.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 123750182
Change-Id: I3f24c8f4f677e2e40503dbab347df1eb031b4132
diff --git a/libartbase/base/string_view_cpp20.h b/libartbase/base/string_view_cpp20.h
new file mode 100644
index 0000000..2c11a2f
--- /dev/null
+++ b/libartbase/base/string_view_cpp20.h
@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ART_LIBARTBASE_BASE_STRING_VIEW_CPP20_H_
+#define ART_LIBARTBASE_BASE_STRING_VIEW_CPP20_H_
+
+#include <string_view>
+
+namespace art {
+
+// Replacement functions for std::string_view::starts_with(), ends_with()
+// which shall be available in C++20.
+#if __cplusplus >= 202000L
+#error "When upgrading to C++20, remove this error and file a bug to remove this workaround."
+#endif
+
+inline bool StartsWith(std::string_view sv, std::string_view prefix) {
+  return sv.substr(0u, prefix.size()) == prefix;
+}
+
+inline bool EndsWith(std::string_view sv, std::string_view suffix) {
+  return sv.size() >= suffix.size() && sv.substr(sv.size() - suffix.size()) == suffix;
+}
+
+}  // namespace art
+
+#endif  // ART_LIBARTBASE_BASE_STRING_VIEW_CPP20_H_