[flang][Lower] Explicitly instantiate all templates from Utils.h

This is intended to reduce the memory usage while compiling flang sources.
Specifically, this reduced the peak memory use when compiling OpenMP.cpp
(on my machine) from 4.9g to 3.6g.

The change was to only leave template declarations in the header file and
explicitly instantiate every case that was previously instantiated
implicitly. If a new implicit instantiation appears later on, it will
cause a compilation error (and the explicit instantiation will need to be
added manually).

Most of the instantiation declarations are generated by an included python
script (about 6000 cases). The remaining cases (~1000) are listed
explicitly.

Author: kparzysz