[ADT] Work around MSVC bug affecting `get(enumerator_result)`

This happened on a small number of MSVC releases (19.31.31xxx, Visual Studio 2022 17.1.x), and worked fine on everything else.

The issue seemed to be related to return type deduction on a function with and `if constexpr`; the compiler got confused and deduced different function return type from the type of the return statement.

The workaround is to split `get` into two functions using `enable_if`.

Reviewed By: dstuttard

Differential Revision: https://reviews.llvm.org/D146893