[Support/Hash functions] Change the final() and result() of the hashing functions to return an array of bytes

Returning `std::array<uint8_t, N>` is better ergonomics for the hashing functions usage, instead of a `StringRef`:

* When returning `StringRef`, client code is "jumping through hoops" to do string manipulations instead of dealing with fixed array of bytes directly, which is more natural
* Returning `std::array<uint8_t, N>` avoids the need for the hasher classes to keep a field just for the purpose of wrapping it and returning it as a `StringRef`

As part of this patch also:

* Introduce `TruncatedBLAKE3` which is useful for using BLAKE3 as the hasher type for `HashBuilder` with non-default hash sizes.
* Make `MD5Result` inherit from `std::array<uint8_t, 16>` which improves & simplifies its API.

Differential Revision: https://reviews.llvm.org/D123100

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -4387,7 +4387,7 @@ void ModuleBitcodeWriter::writeModuleHash(size_t BlockStartPos) {
     uint32_t Vals[5];
     Hasher.update(ArrayRef<uint8_t>((const uint8_t *)&(Buffer)[BlockStartPos],
                                     Buffer.size() - BlockStartPos));
-    StringRef Hash = Hasher.result();
+    std::array<uint8_t, 20> Hash = Hasher.result();
     for (int Pos = 0; Pos < 20; Pos += 4) {
       Vals[Pos / 4] = support::endian::read32be(Hash.data() + Pos);
     }