[libc++] Update clang-format to C++20.

[Mordante]

Summary

We now use clang-format-13 which has the option SpacesInAngles. This
allows us to switch the default language version to C++20, which should
avoid breaking code when formatting due to the adding of whitespace.
For example u8"foo" no longer is formatted as u8 "foo".

libcxx/.clang-format

 BasedOnStyle: LLVM
 
 ---
 Language: Cpp
-Standard: Cpp03
+Standard: c++20
+SpacesInAngles: Leave
 
 AlwaysBreakTemplateDeclarations: true
 PointerAlignment: Left
 
 # Disable formatting options which may break tests.
 SortIncludes: false
 ReflowComments: false
 
 # libc++'s preferred indentions of preprocessor statements.
 IndentPPDirectives: AfterHash
 
 # libc++ has some long names so we need more than the 80 column limit imposed by LLVM style, for sensible formatting
 ColumnLimit: 120
 ---