[DWARFASTParserClang] Start with member offset of 0 for members of union types.

Summary:
GCC does not emit DW_AT_data_member_location for members of a union.
Starting with a 0 value for member locations helps is reading union types
in such cases.

Reviewers: clayborg

Subscribers: ldrumm, lldb-commits

Differential Revision: http://reviews.llvm.org/D18008

llvm-svn: 263085