[IRGen] Fix misalignment of conditional invertible requirement counts

[Azoy]

The conditional invertible protocol set is alone as a 16 bit slot in the generic context, but items here are expected to be 4 bytes large, so some padding needs to accompany this value. If the number of invertible protocols that we're conditional on is even, we can stick a 0 after all of the requirement counts. Also, we were using countBitsUsed to determine how many requirement counts there were, but this is really testing the first bit in [1, 64] that is turned on instead of actually saying how many bits are turned on.

[Azoy]

@swift-ci please smoke test

[DougGregor]

Very nice find

[Azoy]

@swift-ci please smoke test Windows

[Azoy]

@swift-ci please smoke test Windows

[Azoy]

@swift-ci please smoke test

[Azoy]

@swift-ci please smoke test

[Azoy]

@swift-ci please smoke test

[Azoy]

@swift-ci please smoke test

[hjyamauchi]

@Azoy I haven't precisely reproduced but this PR might have caused this build error on windows ARM64 (it is fine on X86_64):

   Creating library lib\_InternalSwiftStaticMirror.lib and object lib\_InternalSwiftStaticMirror.exp
swiftStaticMirror.lib(ObjectFileContext.cpp.obj) : error LNK2019: unresolved external symbol __popcnt referenced in function "protected: unsigned int __cdecl swift::TrailingGenericContextObjects<struct swift::TargetAnonymousContextDescriptor<struct swift::External<struct swift::NoObjCInterop<struct swift::RuntimeTarget<4> > > >,struct swift::TargetGenericContextDescriptorHeader,struct swift::TargetMangledContextName<struct swift::External<struct swift::NoObjCInterop<struct swift::RuntimeTarget<4> > > > >::getNumConditionalInvertibleProtocolsRequirementCounts(void)const " (?getNumConditionalInvertibleProtocolsRequirementCounts@?$TrailingGenericContextObjects@U?$TargetAnonymousContextDescriptor@U?$External@U?$NoObjCInterop@U?$RuntimeTarget@$03@swift@@@swift@@@swift@@@swift@@UTargetGenericContextDescriptorHeader@2@U?$TargetMangledContextName@U?$External@U?$NoObjCInterop@U?$RuntimeTarget@$03@swift@@@swift@@@swift@@@2@@swift@@IEBAIXZ)

bin\_InternalSwiftStaticMirror.dll : fatal error LNK1120: 1 unresolved externals

It seems like __popcnt isn't available on Windows ARM64 according to https://developercommunity.visualstudio.com/t/-popcnt-popcnt64-intrinsics-not-provided-for-aarch/344160#T-N637429

I think we could use this from llvm rather than rolling our own:

llvm/include/llvm/ADT/bit.h:[[nodiscard]] inline int popcount(T Value) noexcept {

[Azoy]

@hjyamauchi that header isn't available to the runtime unfortunately. We can just manually define our own popcount like I did here: #73384

[hjyamauchi]

@Azoy Alejandro, gotcha. Thanks for the fix!