Backport 26068c6e6032 to release/15.x

[DimitryAndric]

This fixes an issue when compiling libc++ with gcc, as described in https://reviews.llvm.org/D135469:

[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.

[DimitryAndric]

/cherry-pick 26068c6

[llvmbot]

Failed to cherry-pick: 26068c6

https://github.com/llvm/llvm-project/actions/runs/3526199683

Please manually backport the fix and push it to your github fork. Once this is done, please add a comment like this:

/branch <user>/<repo>/<branch>

[DimitryAndric]

Hm it looks like this also requires 5fab33a, which changes around of lot of "typename foo" with "__foo". @philnik777 @ldionne is that a too-big change for release/15.x ? Otherwise, I'll tweak the cherry-pick to use the old syntax.

[philnik777]

I don't think we should backport 5fab33a. It's quite a large change.

[bsdjhb]

/branch bsdjhb/llvm-project/type_traits_15.x

[bsdjhb]

Oops, I hadn't seen the conversation, I fixed the cherry-pick to use the old syntax in the branch above.

[DimitryAndric]

I think I arrived at the same syntax here: DimitryAndric@e70c9c4, i.e. add_pointer_t<remove_extent_t<_Up> >.