[DWARFASTParserClang] Factor out structure-like type parsing, NFC

Split out the logic to parse structure-like types into a separate
function, in an attempt to reduce the complexity of ParseTypeFromDWARF.

Inspired by discussion in https://reviews.llvm.org/D68130.

Differential Revision: https://reviews.llvm.org/D68422

llvm-svn: 373927