[libc++] Consistently replace `std::` qualification with `_VSTD::` or nothing. NFCI.

I used a lot of `git grep` to find places where `std::` was being used
outside of comments and assert-messages. There were three outcomes:

- Qualified function calls, e.g. `std::move` becomes `_VSTD::move`.
    This is the most common case.

- Typenames that don't need qualification, e.g. `std::allocator` becomes `allocator`.
    Leaving these as `_VSTD::allocator` would also be fine, but I decided
    that removing the qualification is more consistent with existing practice.

- Names that specifically need un-versioned `std::` qualification,
    or that I wasn't sure about. For example, I didn't touch any code in
    <atomic>, <math.h>, <new>, or any ext/ or experimental/ headers;
    and I didn't touch any instances of `std::type_info`.

In some deduction guides, we were accidentally using `class Alloc = typename std::allocator<T>`,
despite `std::allocator<T>`'s type-ness not being template-dependent.
Because `std::allocator` is a qualified name, this did parse as we intended;
but what we meant was simply `class Alloc = allocator<T>`.

Differential Revision: https://reviews.llvm.org/D92250

GitOrigin-RevId: d586f92c9456a972ee475a021525c0522b89587b
diff --git a/include/cmath b/include/cmath
index 0901a23..138ae6f 100644
--- a/include/cmath
+++ b/include/cmath
@@ -660,8 +660,8 @@
 template <class _IntT, class _RealT>
 _LIBCPP_INLINE_VISIBILITY
 _IntT __clamp_to_integral(_RealT __r) _NOEXCEPT {
-  using _Lim = std::numeric_limits<_IntT>;
-  const _IntT _MaxVal = std::__max_representable_int_for_float<_IntT, _RealT>();
+  using _Lim = numeric_limits<_IntT>;
+  const _IntT _MaxVal = __max_representable_int_for_float<_IntT, _RealT>();
   if (__r >= ::nextafter(static_cast<_RealT>(_MaxVal), INFINITY)) {
     return _Lim::max();
   } else if (__r <= _Lim::lowest()) {