Silence MSVC warnings; NFC

After the changes to DynamicRecursiveASTVisitor, we started getting
several warnings from MSVC like:

warning C4661: 'bool clang::DynamicRecursiveASTVisitorBase<false>::WalkUpFromNamedDecl(clang::NamedDecl *)':
no suitable definition provided for explicit template instantiation request

These changes silence the warnings by providing a definition for those
functions.

Author: AaronBallman

clang/include/clang/AST/DynamicRecursiveASTVisitor.h

@@ -254,6 +254,7 @@ template <bool IsConst> class DynamicRecursiveASTVisitorBase {
   virtual bool Traverse##CLASS##Decl(MaybeConst<CLASS##Decl> *D);
 #include "clang/AST/DeclNodes.inc"
 
+#define ABSTRACT_DECL(DECL)
 #define DECL(CLASS, BASE)                                                      \
   bool WalkUpFrom##CLASS##Decl(MaybeConst<CLASS##Decl> *D);                    \
   virtual bool Visit##CLASS##Decl(MaybeConst<CLASS##Decl> *D) { return true; }
@@ -275,6 +276,7 @@ template <bool IsConst> class DynamicRecursiveASTVisitorBase {
   virtual bool Traverse##CLASS##Type(MaybeConst<CLASS##Type> *T);
 #include "clang/AST/TypeNodes.inc"
 
+#define ABSTRACT_TYPE(CLASS, BASE)
 #define TYPE(CLASS, BASE)                                                      \
   bool WalkUpFrom##CLASS##Type(MaybeConst<CLASS##Type> *T);                    \
   virtual bool Visit##CLASS##Type(MaybeConst<CLASS##Type> *T) { return true; }

clang/lib/AST/DynamicRecursiveASTVisitor.cpp

@@ -221,7 +221,7 @@ template <bool Const> struct Impl : RecursiveASTVisitor<Impl<Const>> {
     return Visitor.Traverse##CLASS##Decl(D);                                   \
   }
 #include "clang/AST/DeclNodes.inc"
-
+#define ABSTRACT_DECL(DECL)
 #define DECL(CLASS, BASE)                                                      \
   bool Visit##CLASS##Decl(CLASS##Decl *D) {                                    \
     return Visitor.Visit##CLASS##Decl(D);                                      \
@@ -246,6 +246,7 @@ template <bool Const> struct Impl : RecursiveASTVisitor<Impl<Const>> {
   }
 #include "clang/AST/TypeNodes.inc"
 
+#define ABSTRACT_TYPE(CLASS, BASE)
 #define TYPE(CLASS, BASE)                                                      \
   bool Visit##CLASS##Type(CLASS##Type *T) {                                    \
     return Visitor.Visit##CLASS##Type(T);                                      \