[LTO] An option to disable automatic bitcode upgrade

Bitcode upgrade automatically takes in place when the current compiler version doesn't match the one building the input modules. The upgrade is very expansive for large applications and unnecessary when users are sure the two compilers do not have a version issue, even if their versions mismatch literally. An optional to disable the automatic bitcode upgrade will be handy in such case.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D109869