Include types when a definition's type differs from a prior declaration.

This is some really old code (took me a while to find the test cases) & the
diagnostic text is slightly incorrect (it should really only apply to
re/declarations/, redefinitions are an error regardless of whether the types
match). Not sure if anyone cares about it, though.

For now this just makes the diagnostic more clear in less obvious cases where
the type of a declaration might not be explicitly written (eg: because it
uses decltype)

llvm-svn: 164313