CodeGenAction::ExecuteAction(): check for invalid LLVM source locations

Add sign checks to deal with the fact that IR parser line/column pairs are
signed integers and sometimes invalid.

The crash path is potentially triggered by corrupt '.bc' files in practice,
though I don't have a binary input test case that can be checked-in right now.

(Unfortunately the backend itself crashes on various ill-formed '.bc' inputs so
this bandage isn't as helpful as it appears yet.)

llvm-svn: 212007