[libc++] Add clang-20 to failing tests on Windows (#100119)

Summary:
After we switched to LLVM version 20, some libc++ tests started failing
on Windows. This patch adds the clang-20 condition to XFAIL to fix the
issue. The way that these tests are excluded from Windows are fragile
and need to be updated every time we bump the LLVM version.

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Differential Revision: https://phabricator.intern.facebook.com/D60251768

Author: gulfemsavrun

libcxx/test/libcxx/fuzzing/random.pass.cpp

@@ -8,7 +8,7 @@
 
 // This test fails because Clang no longer enables -fdelayed-template-parsing
 // by default on Windows with C++20 (#69431).
-// XFAIL: msvc && (clang-18 || clang-19)
+// XFAIL: msvc && (clang-18 || clang-19 || clang-20)
 
 // UNSUPPORTED: c++03, c++11
 

libcxx/test/std/depr/depr.c.headers/math_h.pass.cpp

@@ -8,7 +8,7 @@
 
 // This test fails because Clang no longer enables -fdelayed-template-parsing
 // by default on Windows with C++20 (#69431).
-// XFAIL: msvc && (clang-18 || clang-19)
+// XFAIL: msvc && (clang-18 || clang-19 || clang-20)
 
 // <math.h>
 

libcxx/test/std/numerics/c.math/cmath.pass.cpp

@@ -8,7 +8,7 @@
 
 // This test fails because Clang no longer enables -fdelayed-template-parsing
 // by default on Windows with C++20 (#69431).
-// XFAIL: msvc && (clang-18 || clang-19)
+// XFAIL: msvc && (clang-18 || clang-19 || clang-20)
 
 // <cmath>