Prevent vain lldb::user_id_t 0xffffffff lookups

I have found LLDB commonly looks up lldb::user_id_t 0xffffffff failing to find
its DIE. One would rather expect LLDB_INVALID_UID == 0xffffffffffffffff in such
case.

DWARFASTParserClang.cpp:495
492             type_sp.reset(
493                 new Type(die.GetID(), dwarf, type_name_const_str, byte_size, NULL,
494                          DIERef(encoding_uid).GetUID(dwarf), encoding_data_type,
495                          &decl, clang_type, resolve_state));
encoding_uid = (DWARFFormValue) {m_cu = 0x0, m_form = 0, m_value = {value = {uval = 0, sval = 0, cstr = 0x0}, data = 0x0}}
-> DIERef::DIERef(const DWARFFormValue &form_value = {m_cu = 0x0, m_form = 0, m_value = {value = {uval = 0, sval = 0, cstr = 0x0}, data = 0x0}})
-> (DIERef) {cu_offset = 0xffffffff, die_offset = 0xffffffff}
-> lldb::user_id_t DIERef::GetUID(SymbolFileDWARF *dwarf = 0x61d00000b480) const
-> Type::Type(lldb::user_id_t encoding_uid = 0xffffffff)
But 0xffffffff !=
#define LLDB_INVALID_UID UINT64_MAX

Differential revision: https://reviews.llvm.org/D37492

llvm-svn: 319580