Driver: fix the Windows build after #28543

Repair the VS2017 build after #28543.  It would fail to convert the
dependency set's keys into a vector of StringRefs:

```
error C2678: binary '*': no operator found which takes a left-hand operand of type 'const _Iter' (or there is no acceptable conversion)
        with [
            _Iter=llvm::StringMapKeyIterator<char>
        ]
C:\agent\_work\2\s\toolchain\llvm\include\llvm/ADT/StringMap.h(549): note: could be 'llvm::StringRef &llvm::StringMapKeyIterator<ValueTy>::operator *(void)'
        with
        [
            ValueTy=char
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\xmemory(217): note: while trying to match the argument list '(const _Iter)'
        with
        [
            _Iter=llvm::StringMapKeyIterator<char>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\vector(1823): note: see reference to function template instantiation '_FwdIt *std::_Uninitialized_copy<_Iter,llvm::StringRef*,std::allocator<_Ty>>(const _InIt,const _InIt,_FwdIt,_Alloc &)' being compiled
        with
        [
            _FwdIt=llvm::StringRef *,
            _Iter=llvm::StringMapKeyIterator<char>,
            _Ty=llvm::StringRef,
            _InIt=llvm::StringMapKeyIterator<char>,
            _Alloc=std::allocator<llvm::StringRef>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\vector(708): note: see reference to function template instantiation 'llvm::StringRef *std::vector<llvm::StringRef,std::allocator<_Ty>>::_Ucopy<_Iter>(_Iter,_Iter,llvm::StringRef *)' being compiled
        with
        [
            _Ty=llvm::StringRef,
            _Iter=llvm::StringMapKeyIterator<char>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\vector(708): note: see reference to function template instantiation 'llvm::StringRef *std::vector<llvm::StringRef,std::allocator<_Ty>>::_Ucopy<_Iter>(_Iter,_Iter,llvm::StringRef *)' being compiled
        with
        [
            _Ty=llvm::StringRef,
            _Iter=llvm::StringMapKeyIterator<char>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\vector(723): note: see reference to function template instantiation 'void std::vector<llvm::StringRef,std::allocator<_Ty>>::_Range_construct_or_tidy<_Iter>(_Iter,_Iter,std::forward_iterator_tag)' being compiled
        with
        [
            _Ty=llvm::StringRef,
            _Iter=llvm::StringMapKeyIterator<char>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\vector(723): note: see reference to function template instantiation 'void std::vector<llvm::StringRef,std::allocator<_Ty>>::_Range_construct_or_tidy<_Iter>(_Iter,_Iter,std::forward_iterator_tag)' being compiled
        with
        [
            _Ty=llvm::StringRef,
            _Iter=llvm::StringMapKeyIterator<char>
        ]
C:\agent\_work\2\s\toolchain\swift\lib\Driver\Compilation.cpp(1571): note: see reference to function template instantiation 'std::vector<llvm::StringRef,std::allocator<_Ty>>::vector<IteratorT,void>(_Iter,_Iter,const _Alloc &)' being compiled
        with
        [
            _Ty=llvm::StringRef,
            IteratorT=llvm::StringMapKeyIterator<char>,
            _Iter=llvm::StringMapKeyIterator<char>,
            _Alloc=std::allocator<llvm::StringRef>
        ]
C:\agent\_work\2\s\toolchain\swift\lib\Driver\Compilation.cpp(1571): note: see reference to function template instantiation 'std::vector<llvm::StringRef,std::allocator<_Ty>>::vector<IteratorT,void>(_Iter,_Iter,const _Alloc &)' being compiled
        with
        [
            _Ty=llvm::StringRef,
            IteratorT=llvm::StringMapKeyIterator<char>,
            _Iter=llvm::StringMapKeyIterator<char>,
            _Alloc=std::allocator<llvm::StringRef>
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\xmemory(217): error C2100: illegal indirection
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\\include\xmemory(217): error C2062: type 'unknown-type' unexpected
```

Attempt to make it more explicit with a `std::copy`.

Author: compnerd

lib/Driver/Compilation.cpp

@@ -1568,7 +1568,10 @@ namespace driver {
       if (Comp.getEnableExperimentalDependencies())
         return getExpDepGraph(forRanges).getExternalDependencies();
       const auto deps = getDepGraph(forRanges).getExternalDependencies();
-      return std::vector<StringRef>(deps.begin(), deps.end());
+      std::vector<StringRef> Dependencies;
+      std::copy(std::begin(deps), std::end(deps),
+                std::back_inserter(Dependencies));
+      return Dependencies;
     }
 
     template <unsigned N>