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/runtime/class_linker_test.cc b/runtime/class_linker_test.cc
index 2f37123..a2775d2 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -1254,7 +1254,7 @@
   EXPECT_TRUE(K->IsAssignableFrom(B.Get()));
   EXPECT_TRUE(J->IsAssignableFrom(B.Get()));
 
-  const Signature void_sig = I->GetDexCache()->GetDexFile()->CreateSignature("()V");
+  const std::string_view void_sig("()V");
   ArtMethod* Ii = I->FindClassMethod("i", void_sig, kRuntimePointerSize);
   ArtMethod* Jj1 = J->FindClassMethod("j1", void_sig, kRuntimePointerSize);
   ArtMethod* Jj2 = J->FindClassMethod("j2", void_sig, kRuntimePointerSize);