[lldb] Fix initialization of m_debug_cu_index_map

SymbolFileDWARFDwp contains m_debug_cu_index_map which was previously 
initialized incorrectly: before m_debug_cu_index.parse 
is called m_debug_cu_index is empty, thus 
the map was not actually getting populated properly. 
This diff moves this step into a private helper method
and calls it after m_debug_cu_index.parse inside SymbolFileDWARFDwp::Create.

Test plan:

Build a toy test example 
main.cpp
clang -gsplit-dwarf -g -O0 main.cpp -o main.exe
llvm-dwp -e main.exe -o main.exe.dwp
Build LLDB with ENABLE_DEBUG_PRINTF set.
Run: lldb -- ./main.exe
Check that the indexes are now correct 
(before this change they were empty)
Check that debugging works 
(setting breakpoints, printing local variables (this was not working before))

Differential revision: http://reviews.llvm.org/D38492

llvm-svn: 314832