[libc++] Implement P1004R2 (constexpr std::vector)

Reviewed By: #libc, ldionne

Spies: mgorny, var-const, ormris, philnik, miscco, hiraditya, steven_wu, jkorous, ldionne, christof, libcxx-commits

Differential Revision: https://reviews.llvm.org/D68365

Author: philnik777

libcxx/docs/ReleaseNotes.rst

@@ -38,6 +38,8 @@ What's New in Libc++ 16.0.0?
 Implemented Papers
 ------------------
 
+- P1004R2 (Making ``std::vector`` constexpr)
+
 Improvements and New Features
 -----------------------------
 

libcxx/docs/Status/Cxx20Papers.csv

@@ -107,7 +107,7 @@
 "`P0660R10 <https://wg21.link/P0660R10>`__","LWG","Stop Token and Joining Thread, Rev 10","Cologne","",""
 "`P0784R7 <https://wg21.link/P0784R7>`__","CWG","More constexpr containers","Cologne","|Complete|","12.0"
 "`P0980R1 <https://wg21.link/P0980R1>`__","LWG","Making std::string constexpr","Cologne","|Complete|","15.0"
-"`P1004R2 <https://wg21.link/P1004R2>`__","LWG","Making std::vector constexpr","Cologne","",""
+"`P1004R2 <https://wg21.link/P1004R2>`__","LWG","Making std::vector constexpr","Cologne","|Complete|","15.0"
 "`P1035R7 <https://wg21.link/P1035R7>`__","LWG","Input Range Adaptors","Cologne","",""
 "`P1065R2 <https://wg21.link/P1065R2>`__","LWG","Constexpr INVOKE","Cologne","|Complete|","12.0"
 "`P1135R6 <https://wg21.link/P1135R6>`__","LWG","The C++20 Synchronization Library","Cologne","|Complete|","11.0"

libcxx/include/__algorithm/fill.h

@@ -20,6 +20,8 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// fill isn't specialized for std::memset, because the compiler already optimizes the loop to a call to std::memset.
+
 template <class _ForwardIterator, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 void

libcxx/include/__algorithm/fill_n.h

@@ -19,6 +19,8 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// fill_n isn't specialized for std::memset, because the compiler already optimizes the loop to a call to std::memset.
+
 template <class _OutputIterator, class _Size, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator