Diagnose mixing of tags. For example, for:

struct blah * P;
union blah *P2;

we now emit:

t.c:2:1: error: redefinition of 'blah' with tag that does not match previous use
union blah *P2;
^
t.c:1:8: error: previous use is here
struct blah * P;
       ^

llvm-svn: 39275