Function ABI handling rework meta issue

[PathogenDavid]

The following issues should all be closed by function-arrangement-rework when it is merged into main:

• Don't leak return buffer semantics outside of trampolines #35
• Improve implicit calling convention parameter handling  #53
• Entries in child vtables should not use the base type for this #103
• Handle functions with incomplete return/parameter types. #134
• Revisit MustBePassedByReference(ClangType)'s implementation #135
• Parameter and return types are essentially duplicated in two places for virtual methods #147
• Add helper for lazy-bound references like TranslatedTypeReference #150
• Function pointers requiring return buffer semantics are not handled correctly #186
• Records with constructors are not detected as return by reference correctly in the Microsoft ABI #187

This effort also contributes a lot to #108

Remaining work before merge to main:

• Write tests for ABI handling of function pointers
• Test with OpenCV
• Fix booleans (and maybe char?) in vtables
• Figure out how to work around STL requiring Clang 11 for CI purposes (and maybe for non-CI purposes too since it's annoying to work around. it.) - Upgrade to Clang 12 and resolve forward-compatibility issue with MSVC STL #176
• Remove obsoleted stuff. (The changes for this ended up breaking some API stuff so might as well rip off the bandaid.)