Make sure we get extension diagnostics for GCC's complex extensions.

Now we emit the following when -pedantic-errors is enabled...

[dylan:~/llvm/tools/clang] admin% ../../Debug/bin/clang complex.c -pedantic-errors
complex.c:4:3: error: ISO C does not support '++'/'--' on complex integer types
  ++x;
  ^ ~
complex.c:9:7: error: ISO C does not support '~' for complex conjugation
  X = ~Y;
      ^
complex.c:10:7: error: ISO C does not support '~' for complex conjugation
  x = ~y;
      ^

llvm-svn: 41362