Fix ambiguous FillVRegs template specializations

The two specializations with <char FirstArgType, char...ArgType> and
<char ...ArgType> are ambiguous after
llvm/llvm-project#100692.  For e.g.,
MaterializeVRegs<'L', 'L'> can match both specializations:
  ArgType = <'L', 'L'>, and
  FirstArgType = 'L', ArgType = <'L'>

The first (now-deleted) specialization can match calls with any number
of template parameters, including zero.  The second specialization
matches calls with one or more parameters.  To avoid the ambiguity,
delete the first specialization and explicitly avoid calls with zero
template parameters.

Bug: http://b/363682086
Test: mmma art with ToT clang; and presubmit
Change-Id: I4a3737b4098180faa1f889c2bba475846acc8a51

Author: pirama-arumuga-nainar

runtime/art_method-inl.h

@@ -150,26 +150,23 @@ constexpr size_t NumberOfVRegs() {
   return sum;
 }
 
-template <char... ArgType>
-inline ALWAYS_INLINE void FillVRegs([[maybe_unused]] uint32_t* vregs,
-                                    [[maybe_unused]] typename ShortyTraits<ArgType>::Type... args)
-    REQUIRES_SHARED(Locks::mutator_lock_) {}
-
 template <char FirstArgType, char... ArgType>
 inline ALWAYS_INLINE void FillVRegs(uint32_t* vregs,
                                     typename ShortyTraits<FirstArgType>::Type first_arg,
                                     typename ShortyTraits<ArgType>::Type... args)
     REQUIRES_SHARED(Locks::mutator_lock_) {
   ShortyTraits<FirstArgType>::Set(vregs, first_arg);
-  FillVRegs<ArgType...>(vregs + ShortyTraits<FirstArgType>::kVRegCount, args...);
+  if constexpr (sizeof...(args) > 0)
+    FillVRegs<ArgType...>(vregs + ShortyTraits<FirstArgType>::kVRegCount, args...);
 }
 
 template <char... ArgType>
 inline ALWAYS_INLINE auto MaterializeVRegs(typename ShortyTraits<ArgType>::Type... args)
     REQUIRES_SHARED(Locks::mutator_lock_) {
   constexpr size_t kNumVRegs = NumberOfVRegs<ArgType...>();
   std::array<uint32_t, kNumVRegs> vregs;
-  FillVRegs<ArgType...>(vregs.data(), args...);
+  if constexpr (sizeof...(args) > 0)
+    FillVRegs<ArgType...>(vregs.data(), args...);
   return vregs;
 }