[clang] Use cwg_index.html from GitHub for DR status page (#90352)

Currently we're using official publication of CWG issue list available
at https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_index.html.
Unfortunately, it's not updated as frequently as we sometimes need. For
instance, recently there was a confusion during review of CWG2149 test
(#90079 (comment)).
This patch changes our `make_cxx_dr_status` script to use issue list
from CWG GitHub repository. I confirmed with CWG chair that this is the
most up-to-date source of information on CWG issues.

Changing the source of issue list uncovered previously unhandled
"tentatively ready" status of an issue. This status is considered
unresolved by the script (like `open`, `drafting`, and `review`), as the
resolution might change during face-to-face CWG meeting.

I also noticed that CWG decided to handle 2561 differently from what
we're doing, so this DR is now considered not available in Clang,
despite being declared as available since 18. CC @cor3ntin.

This patch also brings new issues into our DR list, so if someone was
waiting for a newly-filed issue to appear on our status page, it should
appear with this PR.

clang/test/CXX/drs/cwg2149.cpp

@@ -11,7 +11,7 @@
 // cxx98-error@-1 {{variadic macros are a C99 feature}}
 #endif
 
-namespace cwg2149 { // cwg2149: 3.1 drafting 2024-04
+namespace cwg2149 { // cwg2149: 3.1
 #if __cplusplus <= 201103L
 struct X { int i, j, k; };
 #else

clang/test/CXX/drs/dr20xx.cpp

@@ -90,7 +90,7 @@ namespace cwg2026 { // cwg2026: 11
   }
 }
 
-namespace cwg2049 { // cwg2049: 18 drafting P2308R1
+namespace cwg2049 { // cwg2049: 18
 #if __cplusplus >= 202302L
 template <int* x = {}> struct X {};
 X<> a;

clang/test/CXX/drs/dr21xx.cpp

@@ -175,6 +175,8 @@ void foo() {
 }
 }
 
+// cwg2149 is in cwg2149.cpp
+
 namespace cwg2157 { // cwg2157: 11
 #if __cplusplus >= 201103L
   enum E : int;

clang/test/CXX/drs/dr24xx.cpp

@@ -45,7 +45,7 @@ void fallthrough(int n) {
 #endif
 }
 
-namespace cwg2450 { // cwg2450: 18 review P2308R1
+namespace cwg2450 { // cwg2450: 18
 #if __cplusplus >= 202302L
 struct S {int a;};
 template <S s>
@@ -59,7 +59,7 @@ f<{.a= 0}>();
 #endif
 }
 
-namespace cwg2459 { // cwg2459: 18 drafting P2308R1
+namespace cwg2459 { // cwg2459: 18
 #if __cplusplus >= 202302L
 struct A {
   constexpr A(float) {}

clang/test/CXX/drs/dr25xx.cpp

@@ -130,12 +130,14 @@ struct D3 : B {
 #endif
 
 #if __cplusplus >= 202302L
-namespace cwg2561 { // cwg2561: 18 review 2023-11-09
+namespace cwg2561 { // cwg2561: no
 struct C {
     constexpr C(auto) { }
 };
 void foo() {
     constexpr auto b = [](this C) { return 1; };
+    // FIXME: closure type shouldn't have a conversion function to function
+    //        pointer, because explicit object parameter is present. 
     constexpr int (*fp)(C) = b;
     static_assert(fp(1) == 1);
     static_assert((&decltype(b)::operator())(1) == 1);

clang/test/CXX/drs/dr28xx.cpp

@@ -10,7 +10,7 @@
 // expected-no-diagnostics
 #endif
 
-namespace cwg2847 { // cwg2847: 19
+namespace cwg2847 { // cwg2847: 19 review 2024-03-01
 
 #if __cplusplus >= 202002L