[Support] Introduce the BLAKE3 hashing function implementation

BLAKE3 is a cryptographic hash function that is secure and very performant.
The C implementation originates from https://github.com/BLAKE3-team/BLAKE3/tree/1.3.1/c
License is at https://github.com/BLAKE3-team/BLAKE3/blob/1.3.1/LICENSE

This patch adds:

* `llvm/include/llvm-c/blake3.h`: The BLAKE3 C API
* `llvm/include/llvm/Support/BLAKE3.h`: C++ wrapper of the C API
* `llvm/lib/Support/BLAKE3`: Directory containing the BLAKE3 C implementation files, including the `LICENSE` file
* `llvm/unittests/Support/BLAKE3Test.cpp`: unit tests for the BLAKE3 C++ wrapper

This initial patch contains the pristine BLAKE3 sources, a follow-up patch will introduce
LLVM-specific prefixes to avoid conflicts if a client also links with its own BLAKE3 version.

And here's some timings comparing BLAKE3 with LLVM's SHA1/SHA256/MD5.
Timings include `AVX512`, `AVX2`, `neon`, and the generic/portable implementations.
The table shows the speed-up multiplier of BLAKE3 for hashing 100 MBs:

|        Processor        | SHA1  | SHA256 |  MD5 |
|-------------------------|-------|--------|------|
| Intel Xeon W (AVX512)   | 10.4x |   27x  | 9.4x |
| Intel Xeon W (AVX2)     | 6.5x  |   17x  | 5.9x |
| Intel Xeon W (portable) | 1.3x  |  3.3x  | 1.1x |
|      M1Pro (neon)       | 2.1x  |  4.7x  | 2.8x |
|      M1Pro (portable)   | 1.1x  |  2.4x  | 1.5x |

Differential Revision: https://reviews.llvm.org/D121510