[mlir][NVVM|ROCDL] Explicitly construct the return type in loadBitcodeFiles in (NVVM|ROCDL)Target

Fix a build failure in GCC 7 caused by the function:
`std::optional<SmallVector<std::unique_ptr<llvm::Module>>> loadBitcodeFiles` ,
in `NVVMTarget` & `ROCDLTarget`.
The failure is caused because GCC fails to use the move constructor in
`std::optional` for constructing the return value, which prompts a call to the
deleted copy constructor in `std::unique_ptr`, resulting in a failure.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D157804