Fix DWARFUnit::GetUnitDIEPtrOnly stale pointer

GetUnitDIEPtrOnly() needs to return pointer to the first DIE.
But the first element of m_die_array after ExtractDIEsIfNeeded(true)
may move in memory after later ExtractDIEsIfNeeded(false).

DWARFDebugInfoEntry::collection m_die_array is std::vector,
its data may move during its expansion.

Differential revision: https://reviews.llvm.org/D46810

llvm-svn: 333437