[CoreFoundation] _CFObject: Provide default implementation for _hash(into:)

Without this change, the Hashable synthesizer attempts to add _hash(into:) methods directly on CF types, which (somewhat unsurprisingly) fails with the following assert:

Assertion failed: (!decl->isForeign() && "Use getForeignMetadataLayout()"), function getClassMetadataLayout, file /Users/lorentey/Swift/swift/lib/IRGen/MetadataLayout.cpp, line 86.
0  swift                    0x0000000108e28b08 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x0000000108e29216 SignalHandler(int) + 694
2  libsystem_platform.dylib 0x00007fff587aef5a _sigtramp + 26
3  libsystem_platform.dylib 0x0000000000003628 _sigtramp + 2810529512
4  libsystem_c.dylib        0x00007fff585571ae abort + 127
5  libsystem_c.dylib        0x00007fff5851f1ac basename_r + 0
6  swift                    0x0000000105a2a460 swift::irgen::IRGenModule::getClassMetadataLayout(swift::ClassDecl*) + 96
7  swift                    0x0000000105970dc8 swift::irgen::emitVirtualMethodValue(swift::irgen::IRGenFunction&, llvm::Value*, swift::SILDeclRef, swift::CanTypeWrapper<swift::SILFunctionType>) + 184
8  swift                    0x000000010597121d swift::irgen::emitVirtualMethodValue(swift::irgen::IRGenFunction&, llvm::Value*, swift::SILType, swift::SILDeclRef, swift::CanTypeWrapper<swift::SILFunctionType>, bool) + 685
9  swift                    0x0000000105a01482 swift::SILInstructionVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::SILInstruction*) + 36562
10 swift                    0x00000001059f4e4f (anonymous namespace)::IRGenSILFunction::emitSILFunction() + 6863
11 swift                    0x00000001059f2e1b swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 1371
12 swift                    0x00000001058fccab swift::irgen::IRGenerator::emitLazyDefinitions() + 1051
13 swift                    0x00000001059cf676 performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, swift::SourceFile*, llvm::GlobalVariable**, unsigned int) + 1366
14 swift                    0x00000001059cfc5e swift::performIRGeneration(swift::IRGenOptions&, swift::SourceFile&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, unsigned int, llvm::GlobalVariable**) + 94
15 swift                    0x000000010586d2a9 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 15337
16 swift                    0x0000000105868715 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2869
17 swift                    0x000000010581f300 main + 2832
18 libdyld.dylib            0x00007fff584ab015 start + 1

Author: lorentey

stdlib/public/SDK/CoreFoundation/CoreFoundation.swift

@@ -17,6 +17,9 @@ extension _CFObject {
   public var hashValue: Int {
     return Int(bitPattern: CFHash(self))
   }
+  public func _hash(into hasher: _UnsafeHasher) -> _UnsafeHasher {
+    return hasher.appending(self.hashValue)
+  }
   public static func ==(left: Self, right: Self) -> Bool {
     return CFEqual(left, right)
   }