[AST/ObjC] Make ObjCCategoryImplDecl consistent with ObjCCategoryDecl and use the category name as its DeclName

This also addresses the badness in ObjCCategoryImplDecl's API, which was hiding NamedDecl's APIs with different meaning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297131 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed