<type_traits>: Avoid instantiating a pointer type in std::decay<>.

<type_traits>: Avoid instantiating a pointer type in std::decay<>.

GCC expands the pointer type in this conditional expression even for
template types _Up that are not arrays.  This raises an error when
std::decay<> is used with reference types (as is done in LLVM's
sources).  Using add_pointer<> causes GCC to only instantiate a
pointer type for array types.

A similar change to this commit (albeit reworked due to upstream
changes) has been merged to libc++ in commit
26068c6e60324ed866a1ca2afb5cb5eb0aaf015b.

In file included from /usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/__compare/ordering.h:13,
                 from /usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/__compare/common_comparison_category.h:12,
                 from /usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/tuple:168,
                 from /usr/src/contrib/llvm-project/llvm/include/llvm/ADT/DenseMapInfo.h:20,
                 from /usr/src/contrib/llvm-project/llvm/include/llvm/ADT/DenseMap.h:17,
                 from /usr/src/contrib/llvm-project/llvm/lib/Transforms/Scalar/GVNHoist.cpp:36:
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/type_traits: In instantiation of 'struct std::__1::__decay<llvm::CHIArg&, true>':
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/type_traits:1591:89:   required from 'struct std::__1::decay<llvm::CHIArg&&>'
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/__utility/pair.h:132:16:   required by substitution of 'template<class _Tuple, typename std::__1::enable_if<typename std::__1::conditional<(std::__1::__tuple_like_with_size<_Tuple, 2, typename std::__1::__uncvref<_Tp>::type>::value && (! std::__1::is_same<typename std::__1::decay<_Tp>::type, std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> > >::value)), std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> >::_CheckTupleLikeConstructor, std::__1::__check_tuple_constructor_fail>::type::__enable_implicit<_Tuple>(), void>::type* <anonymous> > constexpr std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> >::pair(_Tuple&&) [with _Tuple = llvm::CHIArg&&; typename std::__1::enable_if<typename std::__1::conditional<(std::__1::__tuple_like_with_size<_Tuple, 2, typename std::__1::__uncvref<_Tp>::type>::value && (! std::__1::is_same<typename std::__1::decay<_Tp>::type, std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> > >::value)), std::__1::pair<llvm::BasicBlock*, llvm::SmallVector<llvm::CHIArg, 2> >::_CheckTupleLikeConstructor, std::__1::__check_tuple_constructor_fail>::type::__enable_implicit<_Tuple>(), void>::type* <anonymous> = <missing>]'
/usr/src/contrib/llvm-project/llvm/lib/Transforms/Scalar/GVNHoist.cpp:892:51:   required from here
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/c++/v1/type_traits:1582:30: error: forming pointer to reference type 'std::__1::remove_extent<llvm::CHIArg&>::type' {aka 'llvm::CHIArg&'}
 1582 |                      >::type type;
      |                              ^~~~

Reviewed by:	dim
Differential Revision:	https://reviews.freebsd.org/D36898