Work around build failure due to GCC 4.8.1 bug. We don't completely understand

the details of the bug, but avoiding overloading llvm::cast with another
function template sidesteps it.

See gcc.gnu.org/PR58022 for details of the bug, and llvm.org/PR26362 for more
backgound on how it manifested in Clang. Patch by Igor Sugak!

llvm-svn: 259598