libc++ header snapshot used by the LumeOS kernel build

Upstream project: LLVM libc++
Upstream repository: https://llvm.googlesource.com/llvm-project
LLVM revision: a3f244e2d5550e110daee562b88bee247b75d66c
Fuchsia revision: f8d670ab4bbb71bfb4d569893ba12fbb8a83bacd
Fuchsia package: fuchsia/third_party/clang/linux-amd64
Fuchsia CIPD instance: OK_SEDKvq0UYv-R79oEeWcc3zd2zud7UQB23IxJkRdoC

Imported files:
  include/c++/v1
  LICENSE
  source_manifest.json

Only headers and provenance/license data are imported.  LumeOS does not
vendor or link the package's libc++.a, libc++abi, or libunwind.  The kernel's
KTL configuration overlay under lib/ktl and compiler builtins are maintained
separately.  In particular, Linux/Fuchsia userland __config_site files from
the prebuilt package are intentionally not imported.