Issue Description:

I have narrowed down my compiler issue to the use of the attribute transparent_union with uint8_t, when passing a value of that union type into a c function. Simply removing the attribute or changing the union's size solves the issue.

In c, I have a function called `myFunction`:

void myFunction(const MyUnion_t inValue) {
// body is irrelevant, can be empty or not
}

In Swift I call this function like so:

public static func test(value: MyUnion_t)  {
    myFunction(value)
}

Note: The real compiler error happens only when I use myFunction. And only when the size of the union is uint8_t.

And here is my union type:

Crashes:

typedef union
{
    uint8_t value; 
} MyUnion_t __attribute__ ( (  __transparent_union__ ) );

Compiles:

typedef union
{
    uint16_t value; // <== Anything other than uint8_t works
} MyUnion_t __attribute__ ( (  __transparent_union__ ) );

or

typedef union
{
    uint8_t value;
} MyUnion_t;

Here's the crash log:

Wrong types for attribute: byval inalloca nest noalias nocapture nonnull readnone readonly signext sret zeroext dereferenceable(1) dereferenceable_or_null(1)
call void @x(%union.MyUnion_t zeroext %5), 76
<unknown>:0: error: fatal error encountered during compilation; please file a bug report with your project and the crash log
<unknown>:0: note: Broken function found, compilation aborted!
0 swift 0x000000011190d59a PrintStackTraceSignalHandler(void*) + 42
1 swift 0x000000011190cd4e SignalHandler(int) + 302
2 libsystem_platform.dylib 0x00007fff7ba54b3d _sigtramp + 29
3 libsystem_platform.dylib 0x00000001153a9bc6 _sigtramp + 2576699558
4 libsystem_c.dylib 0x00007fff7b9131c9 abort + 127
5 swift 0x000000010db92a67 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*)::$1::invoke(void*, std::1::basic_string<char, std::1::char_traits<char>, std::_1::allocator<char> > const&, bool) + 519
6 swift 0x00000001118c11c8 llvm::report_fatal_error(llvm::Twine const&, bool) + 280
7 swift 0x00000001118c10ab llvm::report_fatal_error(char const*, bool) + 43
8 swift 0x0000000111888736 (anonymous namespace)::VerifierLegacyPass::runOnFunction(llvm::Function&) + 54
9 swift 0x000000011182ef9e llvm::FPPassManager::runOnFunction(llvm::Function&) + 446
10 swift 0x00000001118372b3 llvm::legacy::FunctionPassManagerImpl::run(llvm::Function&) + 115
11 swift 0x00000001118371a8 llvm::legacy::FunctionPassManager::run(llvm::Function&) + 392
12 swift 0x000000010dd59198 swift::performLLVM(swift::IRGenOptions&, swift::DiagnosticEngine*, llvm::sys::SmartMutex<false>, llvm::GlobalVariable, llvm::Module*, llvm::TargetMachine*, swift::version::Version const&, llvm::StringRef, swift::UnifiedStatsReporter*) + 4504
13 swift 0x000000010db9f242 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 49170
14 swift 0x000000010db8fd35 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7717
15 swift 0x000000010db35965 main + 1349
16 libdyld.dylib 0x00007fff7b86b08d start + 1
17 libdyld.dylib 0x000000000000005d start + 2222542801