ART: Improve class initializer and constructor verification.

DEX file verifier checks additional properties of class initializers
and constructors:

(i) Names match expected <clinit> / <init>.
(ii) The method descriptor for <clinit> is ()V.
(iii) The return type of <init> is V.
(iV) No other names start with '<'.

Bug:  31313719
Change-Id: I60bffa6561e1bae353f97c42377ea556bfa790af
Test: m test-art-host-gtest-dex_file_verifier_test
diff --git a/runtime/dex_file.h b/runtime/dex_file.h
index 250795b..cb7f174 100644
--- a/runtime/dex_file.h
+++ b/runtime/dex_file.h
@@ -1197,6 +1197,9 @@
     return Signature();
   }
 
+  bool IsVoid() const;
+  uint32_t GetNumberOfParameters() const;
+
   bool operator==(const Signature& rhs) const;
   bool operator!=(const Signature& rhs) const {
     return !(*this == rhs);