Log Message:
include-what-you-use: updated to 0.21

iwyu 0.21 compatible with llvm+clang 17 is released. Major changes:

[iwyu] Improve analysis of type aliases (typedef and using)
[iwyu] Improve analysis of namespace aliases (namespace xyz = foobar)
[iwyu] Improve support for elaborated forward declarations (typedef struct Foo Bar;)
[iwyu] Improve handling of "autocast" and function return types, \ 
particularly with complex template types.
[iwyu] Add new IWYU pragma: always_keep, which lets a header announce that it \ 
should always be kept wherever included
[iwyu] Automatically use builtin libc++ mappings if libc++ is the active \ 
standard library
[mappings] Improve mappings for libc++ and posix headers