Make integral-o-pointer conversions in SFINAE illegal.

As reported in PR40362, allowing the conversion from an integral to a
pointer type (despite being illegal in the C++ standard) will cause
surprsing results when testing for certain behaviors in SFINAE.  This
patch converts the error to a SFINAE Error and adds a test to ensure
that it is still a warning in non-SFINAE but an error in it.

Change-Id: I1f475637fa4d83217ae37dc6b5dbf653e118fae4
llvm-svn: 351495