[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/regex b/include/regex
index 028831d..7c5b2fd 100644
--- a/include/regex
+++ b/include/regex
@@ -2475,7 +2475,7 @@
         {
             const bool __in_neg_mask = __traits_.isctype(__ch, __neg_mask_);
           const bool __in_neg_chars =
-              std::find(__neg_chars_.begin(), __neg_chars_.end(), __ch) !=
+              _VSTD::find(__neg_chars_.begin(), __neg_chars_.end(), __ch) !=
               __neg_chars_.end();
           if (!(__in_neg_mask || __in_neg_chars))
           {
@@ -4153,7 +4153,7 @@
                  __first != __last && ( __val = __traits_.value(*__first, 10)) != -1;
                  ++__first)
             {
-                if (__c >= std::numeric_limits<int>::max() / 10)
+                if (__c >= numeric_limits<int>::max() / 10)
                     __throw_regex_error<regex_constants::error_badbrace>();
                 __c *= 10;
                 __c += __val;
@@ -4417,7 +4417,7 @@
             for (++__first;
                     __first != __last && '0' <= *__first && *__first <= '9'; ++__first)
                 {
-                if (__v >= std::numeric_limits<unsigned>::max() / 10)
+                if (__v >= numeric_limits<unsigned>::max() / 10)
                     __throw_regex_error<regex_constants::error_backref>();
                 __v = 10 * __v + *__first - '0';
                 }
@@ -5590,7 +5590,7 @@
                             '0' <= __fmt_first[1] && __fmt_first[1] <= '9')
                         {
                             ++__fmt_first;
-                            if (__idx >= std::numeric_limits<size_t>::max() / 10)
+                            if (__idx >= numeric_limits<size_t>::max() / 10)
                                 __throw_regex_error<regex_constants::error_escape>();
                             __idx = 10 * __idx + *__fmt_first - '0';
                         }
@@ -6401,7 +6401,7 @@
                              regex_constants::match_flag_type __m =
                                                 regex_constants::match_default);
 #if _LIBCPP_STD_VER > 11
-    template <std::size_t _Np>
+    template <size_t _Np>
         regex_token_iterator(_BidirectionalIterator __a,
                              _BidirectionalIterator __b,
                              const regex_type&& __re,