Instead of adding a "flag" (`m` in `...Tgm5`) make it more generic to allow to drop any unused argument.
Add all dropped arguments with a `t<n-1>` (where `<n-1>` is empty for n === 0). For example `...Ttt2g5`.
And replace them with explicit `metatype` instruction in the entry block.
This allows such metatype instructions to be deleted if they are dead.
This was already done for performance-annotated functions. But now do this for all functions.
It is essential that performance-annotated functions are specialized in the same way as other functions.
Because otherwise it can happen that the same specialization has different performance characteristics in different modules.
And it's up to the linker to select one of those ODR functions when linking.
Also, dropping metatype arguments is good for performance and code size in general.
This change also contains a few bug fixes for dropping metatype arguments.
rdar://110509780
