[NFC] Change odd cast-through-unknown behavior to an Optional

This bit of code in the driver uses '~0U' as a sentinel value.
The result is an odd mishmash of casts just to work.  This replaces
it with an optional,  which is a little less crazy looking.
--ehis line, and those below, will be ignored--

M    lib/Driver/Driver.cpp

llvm-svn: 324433