Protect DWARFCompileUnit::m_die_array by a new mutex

Multiple DW_TAG_compile_unit being indexed in a multithreaded way can request
reading of the same DW_TAG_partial_unit.

Unfortunately one cannot detect DWZ file ahead of time to disable such locking
overhead as DWARFCompileUnit::Extract does not read the first DIE which is the
only place one could find early enough if the DWARF file is using any
DW_TAG_partial_unit.

Differential revision: https://reviews.llvm.org/D40470

llvm-svn: 331229