[libc++] Make variables in templates inline (llvm#115785)

The variables are all `constexpr`, which implies `inline`. Since they
aren't `constexpr` in C++03 they're also not `inline` there. Because of
that we define them out-of-line currently. Instead we can use the C++17
extension of `inline` variables, which results in the same weak
definitions of the variables but without having all the boilerplate.

Author: philnik777

libcxx/include/__random/discard_block_engine.h

@@ -43,8 +43,8 @@ class _LIBCPP_TEMPLATE_VIS discard_block_engine {
   typedef typename _Engine::result_type result_type;
 
   // engine characteristics
-  static _LIBCPP_CONSTEXPR const size_t block_size = __p;
-  static _LIBCPP_CONSTEXPR const size_t used_block = __r;
+  static inline _LIBCPP_CONSTEXPR const size_t block_size = __p;
+  static inline _LIBCPP_CONSTEXPR const size_t used_block = __r;
 
 #ifdef _LIBCPP_CXX03_LANG
   static const result_type _Min = _Engine::_Min;
@@ -110,12 +110,6 @@ class _LIBCPP_TEMPLATE_VIS discard_block_engine {
   operator>>(basic_istream<_CharT, _Traits>& __is, discard_block_engine<_Eng, _Pp, _Rp>& __x);
 };
 
-template <class _Engine, size_t __p, size_t __r>
-_LIBCPP_CONSTEXPR const size_t discard_block_engine<_Engine, __p, __r>::block_size;
-
-template <class _Engine, size_t __p, size_t __r>
-_LIBCPP_CONSTEXPR const size_t discard_block_engine<_Engine, __p, __r>::used_block;
-
 template <class _Engine, size_t __p, size_t __r>
 typename discard_block_engine<_Engine, __p, __r>::result_type discard_block_engine<_Engine, __p, __r>::operator()() {
   if (__n_ >= static_cast<int>(__r)) {

libcxx/include/__random/linear_congruential_engine.h

@@ -251,12 +251,12 @@ class _LIBCPP_TEMPLATE_VIS linear_congruential_engine {
   static_assert(_Min < _Max, "linear_congruential_engine invalid parameters");
 
   // engine characteristics
-  static _LIBCPP_CONSTEXPR const result_type multiplier = __a;
-  static _LIBCPP_CONSTEXPR const result_type increment  = __c;
-  static _LIBCPP_CONSTEXPR const result_type modulus    = __m;
+  static inline _LIBCPP_CONSTEXPR const result_type multiplier = __a;
+  static inline _LIBCPP_CONSTEXPR const result_type increment  = __c;
+  static inline _LIBCPP_CONSTEXPR const result_type modulus    = __m;
   _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR result_type min() { return _Min; }
   _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR result_type max() { return _Max; }
-  static _LIBCPP_CONSTEXPR const result_type default_seed = 1u;
+  static inline _LIBCPP_CONSTEXPR const result_type default_seed = 1u;
 
   // constructors and seeding functions
 #ifndef _LIBCPP_CXX03_LANG
@@ -318,22 +318,6 @@ class _LIBCPP_TEMPLATE_VIS linear_congruential_engine {
   operator>>(basic_istream<_CharT, _Traits>& __is, linear_congruential_engine<_Up, _Ap, _Cp, _Np>& __x);
 };
 
-template <class _UIntType, _UIntType __a, _UIntType __c, _UIntType __m>
-_LIBCPP_CONSTEXPR const typename linear_congruential_engine<_UIntType, __a, __c, __m>::result_type
-    linear_congruential_engine<_UIntType, __a, __c, __m>::multiplier;
-
-template <class _UIntType, _UIntType __a, _UIntType __c, _UIntType __m>
-_LIBCPP_CONSTEXPR const typename linear_congruential_engine<_UIntType, __a, __c, __m>::result_type
-    linear_congruential_engine<_UIntType, __a, __c, __m>::increment;
-
-template <class _UIntType, _UIntType __a, _UIntType __c, _UIntType __m>
-_LIBCPP_CONSTEXPR const typename linear_congruential_engine<_UIntType, __a, __c, __m>::result_type
-    linear_congruential_engine<_UIntType, __a, __c, __m>::modulus;
-
-template <class _UIntType, _UIntType __a, _UIntType __c, _UIntType __m>
-_LIBCPP_CONSTEXPR const typename linear_congruential_engine<_UIntType, __a, __c, __m>::result_type
-    linear_congruential_engine<_UIntType, __a, __c, __m>::default_seed;
-
 template <class _UIntType, _UIntType __a, _UIntType __c, _UIntType __m>
 template <class _Sseq>
 void linear_congruential_engine<_UIntType, __a, __c, __m>::__seed(_Sseq& __q, integral_constant<unsigned, 1>) {