[flang][msvc] Work around if constexpr (false) evaluation. NFC.

MSVC tries to expand templates that are in the false-branch of a `if constexpr` construct. In this case, the condition checks whether a tuple has at least one element and then is trying to access it using `std::get<0>`, which fails when the tuple has 0 elements.

The workaround is to extract that case into a separate method.

This patch is part of the series to make flang compilable with MS Visual Studio <http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html>.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D87728