[clangd] Fix crash due to ObjCPropertyDecl

With ObjCPropertyDecl, ASTNode.OrigD can be a ObjCPropertyImplDecl
which is not a NamedDecl, leading to a crash since the code
incorrectly assumes ASTNode.OrigD will always be a NamedDecl.

Change by dgoldman (David Goldman)!

Differential Revision: https://reviews.llvm.org/D56916

llvm-svn: 351941