[Support] Make readNext default to unaligned (#88808)

Without this patch, you would typically use readNext as:

  readNext<uint32_t, llvm::endianness::little, unaligned>(Ptr)

which is quite mouthful.  Since most serialization/deserialization
operations are unaligned accesses, this patch makes the alignment
template parameter default to unaligned, allowing us to say:

  readNext<uint32_t, llvm::endianness::little>(Ptr)

I'm including a few examples of migration in this patch.  I'll do the
rest in a separate patch.

Note that writeNext already has the same trick for the alignment
template parameter.

Author: kazutakahirata

llvm/include/llvm/Support/Endian.h

@@ -80,8 +80,8 @@ template <typename value_type, std::size_t alignment, typename CharT>
   return ret;
 }
 
-template <typename value_type, endianness endian, std::size_t alignment,
-          typename CharT>
+template <typename value_type, endianness endian,
+          std::size_t alignment = unaligned, typename CharT>
 [[nodiscard]] inline value_type readNext(const CharT *&memory) {
   return readNext<value_type, alignment, CharT>(memory, endian);
 }

llvm/include/llvm/Support/OnDiskHashTable.h

@@ -368,14 +368,12 @@ template <typename Info> class OnDiskChainedHashTable {
 
     // 'Items' starts with a 16-bit unsigned integer representing the
     // number of items in this bucket.
-    unsigned Len =
-        endian::readNext<uint16_t, llvm::endianness::little, unaligned>(Items);
+    unsigned Len = endian::readNext<uint16_t, llvm::endianness::little>(Items);
 
     for (unsigned i = 0; i < Len; ++i) {
       // Read the hash.
       hash_value_type ItemHash =
-          endian::readNext<hash_value_type, llvm::endianness::little,
-                           unaligned>(Items);
+          endian::readNext<hash_value_type, llvm::endianness::little>(Items);
 
       // Determine the length of the key and the data.
       const std::pair<offset_type, offset_type> &L =
@@ -473,8 +471,7 @@ class OnDiskIterableChainedHashTable : public OnDiskChainedHashTable<Info> {
         // 'Items' starts with a 16-bit unsigned integer representing the
         // number of items in this bucket.
         NumItemsInBucketLeft =
-            endian::readNext<uint16_t, llvm::endianness::little, unaligned>(
-                Ptr);
+            endian::readNext<uint16_t, llvm::endianness::little>(Ptr);
       }
       Ptr += sizeof(hash_value_type); // Skip the hash.
       // Determine the length of the key and the data.