[lldb][NFC] Extract single member parsing out of DWARFASTParserClang::ParseChildMembers

ParseChildMembers does a few things, only one part is actually parsing a single
member. This extracts the member parsing logic into its own function.

This commit just moves the code as-is into its own function and forwards the parameters/
local variables to it, which means it should be NFC.

The only actual changes to the code are replacing 'break's (and one very curious 'continue'
that behaves like a 'break') with 'return's.