This was done by regex substitution with `sed`. I then manually went
over the entire diff and fixed any incorrect changes.
This diff also changes a lot of `callconv(.C)` to `callconv(.c)`, since
my regex happened to also trigger here. I opted to leave these changes
in, since they *are* a correct migration, even if they're not the one I
was trying to do!
This test has regressed due to a bug in the self-hosted COFF linker.
Jakub recommended disabling it for now, since the COFF linker is being
effectively rewritten, so there is little point in fixing the bug now.
This fixes a bug where, at least with the LLVM backend, `@extern` calls
which had the same name as a normal `extern` in the same Zcu would
result in the `@extern` incorrectly suffixing the identifier `.2`.
Usually, the LLVM backend has a system to change the generated globals
to "collapse" them all together, but it only works if `updateDecl` is
called!
