Highlight the previous underlying enum type when diagnosing a mismatch

enum-scoped.cpp:93:6: error: enumeration redeclared with different underlying type 'short' (was 'int')
  enum Redeclare6 : short;
       ^
enum-scoped.cpp:92:6: note: previous declaration is here
  enum Redeclare6 : int;
       ^            ~~~

The redeclaration source range is still missing but this is a step forward,
potentially edging towards a FixIt.

llvm-svn: 198601