[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/random b/include/random
index 32f9148..3cb2d30 100644
--- a/include/random
+++ b/include/random
@@ -4660,7 +4660,7 @@
     {
         __s_ = _VSTD::sqrt(__mean_);
         __d_ = 6 * __mean_ * __mean_;
-        __l_ = std::trunc(__mean_ - 1.1484);
+        __l_ = _VSTD::trunc(__mean_ - 1.1484);
         __omega_ = .3989423 / __s_;
         double __b1_ = .4166667E-1 / __mean_;
         double __b2_ = .3 * __b1_ * __b1_;
@@ -4692,13 +4692,13 @@
         double __u;
         if (__g > 0)
         {
-            __tx = std::trunc(__g);
+            __tx = _VSTD::trunc(__g);
             if (__tx >= __pr.__l_)
-                return std::__clamp_to_integral<result_type>(__tx);
+                return _VSTD::__clamp_to_integral<result_type>(__tx);
             __difmuk = __pr.__mean_ - __tx;
             __u = __urd(__urng);
             if (__pr.__d_ * __u >= __difmuk * __difmuk * __difmuk)
-                return std::__clamp_to_integral<result_type>(__tx);
+                return _VSTD::__clamp_to_integral<result_type>(__tx);
         }
         exponential_distribution<double> __edist;
         for (bool __using_exp_dist = false; true; __using_exp_dist = true)
@@ -4714,7 +4714,7 @@
                     __u += __u - 1;
                     __t = 1.8 + (__u < 0 ? -__e : __e);
                 } while (__t <= -.6744);
-                __tx = std::trunc(__pr.__mean_ + __pr.__s_ * __t);
+                __tx = _VSTD::trunc(__pr.__mean_ + __pr.__s_ * __t);
                 __difmuk = __pr.__mean_ - __tx;
                 __using_exp_dist = true;
             }
@@ -4758,7 +4758,7 @@
             }
         }
     }
-    return std::__clamp_to_integral<result_type>(__tx);
+    return _VSTD::__clamp_to_integral<result_type>(__tx);
 }
 
 template <class _CharT, class _Traits, class _IntType>