[clang][NFC] Extract Target and AuxTarget creation in CompilerInstance to new function

As @sammccall mentioned in [[ https://reviews.llvm.org/D97109 | D97109 ]], I've extract the logic of creating Target and AuxTarget into a new function called `createTargetAndAuxTarget`.

Since there are many similar code in clang or other related tools, consolidating them into a single function may help others to maintain the logic handling target related things.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D97493