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.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212007 91177308-0d34-0410-b5e6-96231b3b80d8