Relying on features.h is problematic since codebases are free to have
such a header on their search path, which breaks compilation. libc
should instead provide a more standard way of getting __LLVM_LIBC__.
Since __llvm-libc-common.h is included from all libc headers, defining
__LLVM_LIBC__ there ensures that this define is available whenever any
of the standard header is included.
Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:
warning: header guard does not follow preferred style [llvm-header-guard]
This is because many of our header guards start with `__LLVM` rather than
`LLVM`.
To filter just these warnings:
$ ninja -k2000 libc-lint 2>&1 | grep llvm-header-guard
To automatically apply fixits:
$ find libc/src libc/include libc/test -name \*.h | \
xargs -n1 -I {} clang-tidy {} -p build/compile_commands.json \
-checks='-*,llvm-header-guard' --fix --quiet
Some manual cleanup is still necessary as headers that were missing header
guards outright will have them inserted before the license block (we prefer
them after).
