tweak redefinition of a typedef a bit to fix a couple of problems:

1. We had logic in sema to decide whether or not to emit the error
   based on manually checking whether in a system header file.
2. we were allowing redefinitions of typedefs in class scope in C++
   if in header file.
3. there was no way to force typedef redefinitions to be accepted
   by the C compiler, which annoys me when stripping linemarkers out
   of .i files.

The fix is to split the C++ class typedef redefinition path from the
C path, and change the C path to be a warning that normally maps to
error.  This causes it to properly be ignored in system headers, 
etc. and gives us a way to control it.  Passing 
-Wtypedef-redefinition now turns the error into a warning.

One behavior change is that we now diagnose cases where you redefine
a typedef in your .c file that was defined in a header file.  This
seems like reasonable behavior, and the diagnostic now indicates that
it can be controlled with -Wtypedef-redefinition.

llvm-svn: 69391