Issue Description:

Hi Folks -

I'm experiencing a compiler crash which occurs only when archiving with optimizations on.

At the moment, when the compiler segfaults, it gives me the following stack trace:

While running pass #​2022759 SILModuleTransform "PerformanceSILLinker".
0 swift 0x00000001115674ea PrintStackTraceSignalHandler(void*) + 42
1 swift 0x0000000111566cc0 SignalHandler(int) + 352
2 libsystem_platform.dylib 0x00007fff6a7dd5fd _sigtramp + 29
3 libsystem_platform.dylib 000000000000000000 _sigtramp + 2508335648
4 swift 0x000000010dbff165 swift::SILLinkerVisitor::visitProtocolConformance(swift::ProtocolConformanceRef, llvm::Optionalswift::SILDeclRef const&) + 197
5 swift 0x000000010dbfeac9 swift::SILLinkerVisitor::processFunction(swift::SILFunction*) + 825
6 swift 0x000000010dad1740 (anonymous namespace)::SILLinker::run() + 304
7 swift 0x000000010d9c2d09 swift::SILPassManager::execute() + 2329
8 swift 0x000000010d5c74c8 swift::CompilerInstance::performSILProcessing(swift::SILModule*, swift::UnifiedStatsReporter*) + 7912
9 swift 0x000000010d2c3070 performCompileStepsPostSILGen(swift::CompilerInstance&, swift::CompilerInvocation&, std::_1::unique_ptr<swift::SILModule, std::_1::default_deleteswift::SILModule >, bool, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, bool, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1712
10 swift 0x000000010d2bab95 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 55813
11 swift 0x000000010d230b73 main + 1283
12 libdyld.dylib 0x00007fff6a5e4cc9 start + 1

This issue seems to be introduced starting in Swift 5.2 (in the toolchains bundled in Xcode 11.4-11.5). I do not experience the issue building with Xcode 11.3.

Here's a code sample that can be used to reproduce the issue:

protocol A {}
protocol B {}

class T: A, B {}

@objcMembers class U {
    func a() { b(T()) }
    func b(_ p: T & B) { c(p) }
    func c(_ p: A & B) { print(p) }
}