Match also DW_TAG_partial_unit when DW_TAG_compile_unit is matched

Code commonly checks if the parent DIE is DW_TAG_compile_unit.
But DW_TAG_partial_unit also acts as DW_TAG_compile_unit for DWZ
as DWZ is using DW_TAG_imported_unit only at the top unit level.

Differential revision: https://reviews.llvm.org/D40469

llvm-svn: 331194