[Support] Use "inline static" to initialize Registry (NFC) (#160235)

In C++17, we can initialize a static member variable with "inline
static" as part of the class definition.  With this, we can eliminate
the out-of-line static initializers involving boilerplate template
code.

Author: kazutakahirata

llvm/include/llvm/Support/Registry.h

@@ -58,8 +58,8 @@ namespace llvm {
     // declaration causing error C2487 "member of dll interface class may not
     // be declared with dll interface".
     // https://developercommunity.visualstudio.com/t/c2487-in-dllexport-class-with-static-members/69878
-    static node *Head;
-    static node *Tail;
+    static inline node *Head = nullptr;
+    static inline node *Tail = nullptr;
 
   public:
     /// Node in linked list of entries.
@@ -143,19 +143,11 @@ namespace llvm {
 /// Instantiate a registry class.
 #define LLVM_INSTANTIATE_REGISTRY(REGISTRY_CLASS)                              \
   namespace llvm {                                                             \
-  template <typename T>                                                        \
-  typename Registry<T>::node *Registry<T>::Head = nullptr;                     \
-  template <typename T>                                                        \
-  typename Registry<T>::node *Registry<T>::Tail = nullptr;                     \
   template class LLVM_ABI_EXPORT Registry<REGISTRY_CLASS::type>;               \
   }
 #else
 #define LLVM_INSTANTIATE_REGISTRY(REGISTRY_CLASS)                              \
   namespace llvm {                                                             \
-  template <typename T>                                                        \
-  typename Registry<T>::node *Registry<T>::Head = nullptr;                     \
-  template <typename T>                                                        \
-  typename Registry<T>::node *Registry<T>::Tail = nullptr;                     \
   template class Registry<REGISTRY_CLASS::type>;                               \
   }
 #endif