[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/complex b/include/complex
index ed84cd4..93b7bb5 100644
--- a/include/complex
+++ b/include/complex
@@ -950,7 +950,7 @@
 complex<_Tp>
 proj(const complex<_Tp>& __c)
 {
-    std::complex<_Tp> __r = __c;
+    complex<_Tp> __r = __c;
     if (__libcpp_isinf_or_builtin(__c.real()) || __libcpp_isinf_or_builtin(__c.imag()))
         __r = complex<_Tp>(INFINITY, copysign(_Tp(0), __c.imag()));
     return __r;