[lldb] Enable debugging of binaries with mixed (splitted/regular) dwarf

Initialize the default value of SymbolFileDWARF uuid with 
the appropriately shifted DW_INVALID_OFFSET constant.
This change fixes the collision in the computation of DIE uid 
(inside DIERef::GetUID) and incorrect CompileUnit lookup
(because of the misleading cu_offset value).

Test plan: make check-lldb

Differential revision: https://reviews.llvm.org/D42563

llvm-svn: 323832