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