[clang] Check AuxTarget exists when creating target in CompilerInstance

D97493 separate target creation out to a single function
`CompilerInstance::createTarget`. However, it would overwrite AuxTarget
even if it has been set.
As @kadircet recommended in D98128, this patch check the existence of
AuxTarget and not overwrite it when it has been set.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D100024