All callers were passing in "a.out" or garbage so a sensible default works fine
here as a cleanup.

This also brings about the possibility of adapting the value based on the
driver's compatibility mode in future.

The setting can still be changed via Driver::DefaultImageName as needed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208926 91177308-0d34-0410-b5e6-96231b3b80d8