Fix non-standard parsing of DWARF 5 Type Unit Headers (7.5.3)

[zadewg]

It appears to me that the current implementation of DWARF 5 Unit Type Header parsing in CU::ReadHeader does not honour the standard.

Section 7.5.1.3 Type Unit Headers reads:

3. unit_type (ubyte)
A 1-byte unsigned integer identifying this unit as a type unit. The value of
this field is DW_UT_type for a non-split type unit (see Section 3.1.4 on
page 68) or DW_UT_split_type for a split type unit.

The current implementation associates DW_UT_split_type with Skeleton and Split Compilation Unit Headers (7.5.1.2), wrongly consuming a dwo_id unit ID an missing the type_signature and type_offset information.

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[zadewg]

@haberman any thoughts on this?