Return a std::unique_ptr from parseInputFile and propagate. NFC.

The memory management in BugPoint is fairly convoluted, so this just unwraps
one layer by changing the return type of functions that always return
owned Modules.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216464 91177308-0d34-0410-b5e6-96231b3b80d8