[clangd][nfc] Give name to a magic constant

[jkorous-apple]

Summary

Since I was reading this code I decided I might just as well polish it a little. It is just preliminary commit for a bug-fix.

[jkorous-apple]

I commited the patch but forgot to quote this review.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@327901 91177308-0d34-0410-b5e6-96231b3b80d8

clangd/index/Index.h

public:
   bool operator==(const SymbolID &Sym) const {
     return HashValue == Sym.HashValue;
   }
   bool operator<(const SymbolID &Sym) const {
     return HashValue < Sym.HashValue;
   }
 
 private:
+  static constexpr unsigned HashByteLength = 20;
+
   friend llvm::hash_code hash_value(const SymbolID &ID) {
     // We already have a good hash, just return the first bytes.
-    static_assert(sizeof(size_t) <= 20, "size_t longer than SHA1!");
+    static_assert(sizeof(size_t) <= HashByteLength, "size_t longer than SHA1!");
     return *reinterpret_cast<const size_t *>(ID.HashValue.data());
   }
   friend llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
                                        const SymbolID &ID);
   friend void operator>>(llvm::StringRef Str, SymbolID &ID);
 
-  std::array<uint8_t, 20> HashValue;
+  std::array<uint8_t, HashByteLength> HashValue;
 };
 
 // Write SymbolID into the given stream. SymbolID is encoded as a 40-bytes
 // hex string.
 llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SymbolID &ID);
 
 // Construct SymbolID from a hex string.
 // The HexStr is required to be a 40-bytes hex string, which is encoded from the