Modified the code so that we exit() with a non-zero value instead of calling

Modified the code so that we exit() with a non-zero value instead of calling
abort() when we can't open the input filename.  The abort() function generates
a core dump if possible and is meant for handling errors so grand that even the
program knows that debugging is necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8415 91177308-0d34-0410-b5e6-96231b3b80d8