[clangd] Fix undefined behavior due to misaligned type cast

[jkorous-apple]

Summary

The current code is casting pointer to a misaligned type which is undefined behavior.

Found by compiling with Undefined Behavior Sanitizer and running tests (check-clang-tools).

AFAIK clang produces the same code for reinterpret_cast<>() and memcpy().

Follow-up to: https://reviews.llvm.org/D44575

[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@327902 91177308-0d34-0410-b5e6-96231b3b80d8

clangd/index/Index.h

public:
   }
 
 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) <= HashByteLength, "size_t longer than SHA1!");
-    return *reinterpret_cast<const size_t *>(ID.HashValue.data());
+    size_t Result;
+    memcpy(&Result, ID.HashValue.data(), sizeof(size_t));
+    return llvm::hash_code(Result);
   }
   friend llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
                                        const SymbolID &ID);
   friend void operator>>(llvm::StringRef Str, SymbolID &ID);
 
   std::array<uint8_t, HashByteLength> HashValue;
 };