[SE-0206][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 assertion below. (This situation is unique to CF types, which are imported with an implicit _CFObject conformance; we usually have an extension context or a non-imported type decl in which to put the derived methods.)

Assertion failed: (!decl->isForeign() && "Use getForeignMetadataLayout()"), function getClassMetadataLayout, file /Users/klorentey/Swift/swift/lib/IRGen/MetadataLayout.cpp, line 83.
0  swift                    0x000000010ccd29c8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x000000010ccd30d6 SignalHandler(int) + 694
2  libsystem_platform.dylib 0x00007fff600e0d9a _sigtramp + 26
3  swift                    0x000000010e04f1ed cmark_strbuf__initbuf + 122440
4  libsystem_c.dylib        0x00007fff5ffa6f79 abort + 127
5  libsystem_c.dylib        0x00007fff5ff6f090 basename_r + 0
6  swift                    0x00000001096e8f95 swift::irgen::IRGenModule::getClassMetadataLayout(swift::ClassDecl*) + 53
7  swift                    0x00000001095b9e25 swift::irgen::emitVirtualMethodValue(swift::irgen::IRGenFunction&, llvm::Value*, swift::SILDeclRef, swift::CanTypeWrapper<swift::SILFunctionType>) + 133
8  swift                    0x00000001095ba252 swift::irgen::emitVirtualMethodValue(swift::irgen::IRGenFunction&, llvm::Value*, swift::SILType, swift::SILDeclRef, swift::CanTypeWrapper<swift::SILFunctionType>, bool) + 690
9  swift                    0x00000001096bef49 swift::SILInstructionVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::SILInstruction*) + 33497
10 swift                    0x00000001096b3067 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 8055
11 swift                    0x00000001095c8d8b swift::irgen::IRGenerator::emitLazyDefinitions() + 1051
12 swift                    0x000000010968eb16 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) + 1382
13 swift                    0x000000010968f05e 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
14 swift                    0x000000010952cfc0 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 15488
15 swift                    0x0000000109528332 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2962
16 swift                    0x00000001094e4848 main + 2328
17 libdyld.dylib            0x00007fff5feecc41 start + 1
Stack dump:
[…]
1.	While emitting IR SIL function "@$SSo10CGColorRefas8HashableSCsACP4hash4intoys6HasherVz_tFTW".
 for 'hash(into:)' in module 'CoreGraphics'

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: inout Hasher) {
+    hasher.combine(self.hashValue)
+  }
   public static func ==(left: Self, right: Self) -> Bool {
     return CFEqual(left, right)
   }