PR28739: Check that integer values fit into 64 bits before extracting them as...

PR28739: Check that integer values fit into 64 bits before extracting them as 64 bit values for pointer arithmetic.

This fixes various ways to tickle an assertion in constant expression
evaluation when using __int128. Longer term, we need to figure out what should
happen here: either any kind of overflow in offset calculation should result in
a non-constant value or we should truncate to 64 bits. In C++11 onwards, we're
effectively already checking for overflow because we strictly enforce array
bounds checks, but even there some forms of overflow can slip past undetected.

llvm-svn: 293568