QVarLengthArray: remove unnecessary exception check

The previous check for a non-throwing copy constructor at:

    if constexpr (IsFwdIt && noexcept(T(*first)))

is not necessary as std::uninitialized_copy provides strong exception
safety.

Additionally, change the phrasing of the overload-resolution \note,
since we're not requiring C++20 std::input_iterator, but the older
C++17 definition.

Amends 7cbdc8abbd.
Amends 2457dd8bd0.

Change-Id: Ie36c8d70dc61aa8cc2a30c9d4110d1beb0d1c2fe
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -796,8 +796,8 @@ Q_OUTOFLINE_TEMPLATE void QVLABase<T>::assign_impl(qsizetype prealloc, void *arr
         ++first;
     }
 
-    qsizetype n = 0;
-    if constexpr (IsFwdIt && noexcept(T(*first))) {
+    qsizetype n;
+    if constexpr (IsFwdIt) {
         dst = std::uninitialized_copy(first, last, dst);
         n = dst - begin();
         if (n > s) // otherwise: readjust 's' in erase() later

src/corelib/tools/qvarlengtharray.qdoc

@@ -109,7 +109,8 @@
     Constructs an array with the contents in the iterator range [\a first, \a last).
 
     This constructor only participates in overload resolution if
-    \c InputIterator models the \c std::input_iterator concept.
+    \c InputIterator meets the requirements of an
+    \l {https://en.cppreference.com/w/cpp/named_req/InputIterator} {LegacyInputIterator}.
 
     The value type of \c InputIterator must be convertible to \c T.
 */
@@ -1016,7 +1017,9 @@
     number of elements in the range exceeds the capacity of the container.
 
     This function overload only participates in overload resolution if
-    \c InputIterator models the \c std::input_iterator concept.
+    \c InputIterator meets the requirements of an
+    \l {https://en.cppreference.com/w/cpp/named_req/InputIterator} {LegacyInputIterator}.
+
     The behavior is undefined if either argument is an iterator into *this.
 */