Implement PR6180, substantially improving the diagnostics we get from

forgetting a ';' at the end of a struct.  For something like:

class c {
}
void foo() {}

we now produce:

t.cc:3:2: error: expected ';' after class
}
 ^
 ;

instead of:

t.cc:4:1: error: cannot combine with previous 'class' declaration specifier
void foo() {}
^
t.cc:2:7: error: 'class c' can not be defined in the result type of a function
class c {
      ^

GCC produces:

t.cc:4: error: new types may not be defined in a return type
t.cc:4: note: (perhaps a semicolon is missing after the definition of ‘c’)
t.cc:4: error: two or more data types in declaration of ‘foo’

I *think* I got the follow set right, but if I forgot anything, we'll start 
getting spurious "expected ';' after class" errors, let me know if you see
any.

llvm-svn: 95042