PR26111: segmentation fault with __attribute__((mode(QI))) on function declaration, by Denis Zobnin

Allow "mode" attribute to be applied to VarDecl, not ValueDecl (which includes FunctionDecl and EnumConstantDecl), emit an error if this attribute is used with function declarations and enum constants.
Differential Revision: http://reviews.llvm.org/D16112

llvm-svn: 257868