[lldb] Specify unsigned underlying type for an enumeration explicitly

[lldb] Specify unsigned underlying type for an enumeration explicitly

The enumeration EntryType is used as a bit field of DebugMacroEntry:
```
  EntryType m_type : 3
```

Since underlying type of enumeration is implementation-dependent, a signed integer is
converted to the 3-bit value by some compilers (MSVC).

That's why a DebugMacroEntry instance that was created with EntryType value > 3 (END_FILE or INDIRECT)
contains incorrect negative value in its m_type data-member.