Fix dumping of characters with non-standard sizes

* Prevent dumping of characters in DumpDataExtractor() with
  item_byte_size bigger than 8 bytes. This case is not supported by the
  code and results in a crash because the code calls
  DataExtractor::GetMaxU64Bitfield() -> GetMaxU64() that asserts for
  byte size > 8 bytes.
* Teach DataExtractor::GetMaxU64(), GetMaxU32(), GetMaxS64() and
  GetMaxU64_unchecked() how to handle byte sizes that are not a multiple
  of 2. This allows DumpDataExtractor() to dump characters and booleans
  with item_byte_size in the interval of [1, 8] bytes. Values that are
  not a multiple of 2 would previously result in a crash because they
  were not handled by GetMaxU64().

llvm-svn: 315444