[clang] Support __typeof_unqual__ in all C modes (#87392)

GCC has added __typeof_unqual__ to allow typeof_unqual to be used in all
C modes (not just C23 and newer), similar to __typeof__ and typeof.

https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=607d9d50ee44163cee621cd991600acaf78c2fee

The Linux kernel would like to start using __typeof_unqual__ to strip
type qualifiers such as address spaces from inputs to macros but cannot
switch to C23 due to compiler version requirements.

Match GCC and allow __typeof_unqual__ in all C modes.

Closes: https://github.com/llvm/llvm-project/issues/76423
Link: https://lore.kernel.org/CAFULd4YG21NdF_qNVBGDtXO6xnaYFeRPvKicB=gpgUUqYE=4jw@mail.gmail.com/