Add isFirstDecl to DecBase too and use it instead of getPreviousDecl() == 0.

Redeclarable already had a isFirstDecl, but it was missing from DeclBase.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193027 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index d232670..3ee9830 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -3531,7 +3531,7 @@
 
   for (unsigned I = 0, N = Redeclarations.size(); I != N; ++I) {
     Decl *First = Redeclarations[I];
-    assert(First->getPreviousDecl() == 0 && "Not the first declaration?");
+    assert(First->isFirstDecl() && "Not the first declaration?");
     
     Decl *MostRecent = First->getMostRecentDecl();