Reduce memory consumption when compiling flang

[kparzysz]

I've been experimenting with various things related to memory usage by template instantiations. This draft will have some commits that I think may be helpful.

cc: @pawosm-arm @tblah

Ignore formatting errors, this is just a draft, but it should work otherwise.

[kparzysz]

Moving all template instantiations from Lower/Support/Utils.h to Lower/Support/Utils.cpp reduced peak memory use when compiling OpenMP.cpp (on my machine) from 4.9g to 3.7g.

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). The change lists all instantiations explicitly, but it could likely be shortened using macros.

[tblah]

I like the idea.

If these are generated from a script, it might be worth committing the script as well so the file can be more easily re-generated.

[kparzysz]

The way it was generated was a combination of

1. Annotating clang to print class and function instantiations (with extra info).
2. Capturing the dump from compiling OpenMP.cpp.
3. Filtering out everything from the dumps except the instantiations' pretty names, e.g. void foo<T>() [T=Fortran::evaluate::Something].
4. Using a python script to plug the T from the list in [] into the template name, then print the resulting string.

Essentially this is a list of things that actually got instantiated. There are some patterns in the output, and 99% of it could be auto-generated, but I don't have any scripts to do that.

[tblah]

Okay that's fine without a script then

[kparzysz]

I'm finishing a python script to generate these instantiations. Should have something later today or tomorrow.

[kparzysz]

The PR with the script: #125216