[libc++] <type_traits>: Avoid instantiating a pointer type.

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.

Reviewed By: #libc, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D135469

Author: bsdjhb

libcxx/include/__type_traits/decay.h

@@ -42,7 +42,7 @@ struct __decay<_Up, true> {
     typedef _LIBCPP_NODEBUG typename conditional
                      <
                          is_array<_Up>::value,
-                         __remove_extent_t<_Up>*,
+                         __add_pointer_t<__remove_extent_t<_Up> >,
                          typename conditional
                          <
                               is_function<_Up>::value,

libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/decay.pass.cpp

@@ -29,10 +29,14 @@ int main(int, char**)
     test_decay<int, int>();
     test_decay<const volatile int, int>();
     test_decay<int*, int*>();
+    test_decay<int&, int>();
+    test_decay<const volatile int&, int>();
     test_decay<int[3], int*>();
     test_decay<const int[3], const int*>();
     test_decay<void(), void (*)()>();
 #if TEST_STD_VER > 11
+    test_decay<int&&, int>();
+    test_decay<const volatile int&&, int>();
     test_decay<int(int) const, int(int) const>();
     test_decay<int(int) volatile, int(int) volatile>();
     test_decay<int(int)  &, int(int)  &>();