[libc++] [libc++abi] Use C++20 standard.

This change is needed to use char8_t when building libc++.
Using the same standard in libc++abi for coherence.

See https://reviews.llvm.org/D91517.

Reviewed By: ldionne, #libc, #libc_abi

Differential Revision: https://reviews.llvm.org/D91691

libcxx/CMakeLists.txt

@@ -514,11 +514,11 @@ remove_flags(-Wno-pedantic -pedantic-errors -pedantic)
 # Required flags ==============================================================
 function(cxx_add_basic_build_flags target)
 
-  # Require C++17 for all targets. C++17 is needed to use aligned allocation
-  # in the dylib.
+  # Require C++20 for all targets. C++17 is needed to use aligned allocation
+  # in the dylib. C++20 is needed to use char8_t.
   set_target_properties(${target} PROPERTIES
-    CXX_STANDARD 17
-    CXX_STANDARD_REQUIRED YES
+    CXX_STANDARD 20
+    CXX_STANDARD_REQUIRED NO
     CXX_EXTENSIONS NO)
 
   # When building the dylib, don't warn for unavailable aligned allocation

libcxxabi/src/CMakeLists.txt

@@ -174,9 +174,9 @@ if (LIBCXXABI_ENABLE_SHARED)
                           CXX_EXTENSIONS
                             OFF
                           CXX_STANDARD
-                            17
+                            20
                           CXX_STANDARD_REQUIRED
-                            ON
+                            OFF
                           COMPILE_FLAGS
                             "${LIBCXXABI_COMPILE_FLAGS}"
                           LINK_FLAGS
@@ -241,9 +241,9 @@ if (LIBCXXABI_ENABLE_STATIC)
                           CXX_EXTENSIONS
                             OFF
                           CXX_STANDARD
-                            17
+                            20
                           CXX_STANDARD_REQUIRED
-                            ON
+                            OFF
                           COMPILE_FLAGS
                             "${LIBCXXABI_COMPILE_FLAGS}"
                           LINK_FLAGS