Unrevert r158887, reverted in r158949, along with a fix for the bug which

resulted in it being reverted. A test for that bug was added in r158950.

Original comment:

If an object (such as a std::string) with an appropriate c_str() member function
is passed to a variadic function in a position where a format string indicates
that c_str()'s return type is desired, provide a note suggesting that the user
may have intended to call the c_str() member.

Factor the non-POD-vararg checking out of DefaultVariadicArgumentPromotion and
move it to SemaChecking in order to facilitate this. Factor the call checking
out of function call checking and block call checking, and extend it to cover
constructor calls too.

Patch by Sam Panzer!

llvm-svn: 159159