toolchain: llvm: Use clang's __INTN_C/__UINTN_C macros for clang 20+

clang recently began providing predefined __INTN_C/__UINTN_C macros
resulting in macro redefinition warnings in toolchain/llvm.h. This
change was landed in clang in late Jan. 2025 and clang/LLVM 20 is the
first official release that has this support (see the PR linked below).

Prefer the definitions provided by clang and avoid redefining these macros
for clang versions 20 and later to avoid said warnings.

Link: https://github.com/llvm/llvm-project/pull/123514



Signed-off-by: Jonathon Penix <jpenix@quicinc.com>