[dsymutil] Don't clone empty CUs

The DWARF standard says that an empty compile unit is not valid:

> Each such contribution consists of a compilation unit header (see
> Section 7.5.1.1 on page 200) followed by a single DW_TAG_compile_unit or
> DW_TAG_partial_unit debugging information entry, together with its
> children.

Therefore we shouldn't clone them in dsymutil.

Differential revision: https://reviews.llvm.org/D57979

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353903 91177308-0d34-0410-b5e6-96231b3b80d8