[SR-11813] DispatchGroup not Equatable on Linux

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Issue Description:

It appears that DispatchGroups are Equatable on MacOS, but not on Linux.

Using the following simple test program (as foo.swift):

import Dispatch
let g1 = DispatchGroup()
let g2 = DispatchGroup()
if g1 == g2 {
    print("groups are equal")
}
else {
    print("groups are NOT equal")
}

On MacOS:

$ swift --version
Apple Swift version 5.1.2 (swiftlang-1100.0.278 clang-1100.0.33.9)
Target: x86_64-apple-darwin18.7.0
$ swift foo.swift 
groups are NOT equal

On Linux (Ubuntu 18.04):

$ swift --version                                                                                                                                                                                                                                                           
Swift version 5.1 (swift-5.1.2-RELEASE)
Target: x86_64-unknown-linux-gnu
$ swift foo.swift                                                                                                                                                                                                                                                           
foo.swift:8:7: error: binary operator '==' cannot be applied to two 'DispatchGroup' operands
if g1 == g2 {
   ~~ ^  ~~
foo.swift:8:7: note: overloads for '==' exist with these partially matching parameter lists: ((), ()), ((A, B), (A, B)), ((A, B, C), (A, B, C)), ((A, B, C, D), (A, B, C, D)), ((A, B, C, D, E), (A, B, C, D, E)), ((A, B, C, D, E, F), (A, B, C, D, E, F)), (Any.Type?, Any.Type?), (AnyHashable, AnyHashable), (AnyIndex, AnyIndex), (AnyKeyPath, AnyKeyPath), (Array<Element>, Array<Element>), (ArraySlice<Element>, ArraySlice<Element>), (Bool, Bool), (Character, Character), (ClosedRange<Bound>, ClosedRange<Bound>), (ClosedRange<Bound>.Index, ClosedRange<Bound>.Index), (CodingUserInfoKey, CodingUserInfoKey), (CollectionDifference<ChangeElement>, CollectionDifference<ChangeElement>), (CollectionDifference<ChangeElement>.Change, CollectionDifference<ChangeElement>.Change), (CollectionDifference<ChangeElement>.Index, CollectionDifference<ChangeElement>.Index), (ContiguousArray<Element>, ContiguousArray<Element>), (Dictionary<Key, Value>.Index, Dictionary<Key, Value>.Index), (Dictionary<Key, Value>.Keys, Dictionary<Key, Value>.Keys), (DispatchQoS, DispatchQoS), (DispatchTime, DispatchTime), (DispatchTimeInterval, DispatchTimeInterval), (DispatchWallTime, DispatchWallTime), (EmptyCollection<Element>, EmptyCollection<Element>), (FlattenSequence<Base>.Index, FlattenSequence<Base>.Index), (FloatingPointClassification, FloatingPointClassification), (FloatingPointRoundingRule, FloatingPointRoundingRule), (FloatingPointSign, FloatingPointSign), (Int, Int), (Int16, Int16), (Int32, Int32), (Int64, Int64), (Int8, Int8), (LazyPrefixWhileSequence<Base>.Index, LazyPrefixWhileSequence<Base>.Index), (ManagedBufferPointer<Header, Element>, ManagedBufferPointer<Header, Element>), (Mirror.DisplayStyle, Mirror.DisplayStyle), (Never, Never), (ObjectIdentifier, ObjectIdentifier), (OpaquePointer, OpaquePointer), (Range<Bound>, Range<Bound>), (Result<Success, Failure>, Result<Success, Failure>), (ReversedCollection<Base>.Index, ReversedCollection<Base>.Index), (Self, Other), (Self, RHS), (Set<Element>, Set<Element>), (Set<Element>.Index, Set<Element>.Index), (String, String), (String.Index, String.Index), (UInt, UInt), (UInt16, UInt16), (UInt32, UInt32), (UInt64, UInt64), (UInt8, UInt8), (Unicode.CanonicalCombiningClass, Unicode.CanonicalCombiningClass), (Unicode.GeneralCategory, Unicode.GeneralCategory), (Unicode.NumericType, Unicode.NumericType), (Unicode.Scalar, Unicode.Scalar), (Unicode.UTF32, Unicode.UTF32), (UnicodeDecodingResult, UnicodeDecodingResult), (Wrapped?, Wrapped?), (Wrapped?, _OptionalNilComparisonType), ([Key : Value], [Key : Value]), (_OptionalNilComparisonType, Wrapped?), (_UIntBuffer<Element>.Index, _UIntBuffer<Element>.Index), (_ValidUTF8Buffer.Index, _ValidUTF8Buffer.Index)
if g1 == g2 {
      ^

Start with the foo.swift reproducer in the issue and compare DispatchGroup equality behavior on Linux with the documented macOS result. Then inspect the libdispatch component for the DispatchGroup API and its platform-specific implementation. Done means the same comparison compiles and produces the expected result on Linux.