[lldb][DWARFASTParserClang][NFC] Extract condition for unnamed bitfield...

[lldb][DWARFASTParserClang][NFC] Extract condition for unnamed bitfield creation into helper function

This patch adds a new private helper
`DWARFASTParserClang::ShouldCreateUnnamedBitfield` which
`ParseSingleMember` whether we should fill the current gap
in a structure layout with unnamed bitfields.

Extracting this logic will allow us to add additional
conditions in upcoming patches without jeoperdizing readability
of `ParseSingleMember`.

We also store some of the boolean conditions in local variables
to make the intent more obvious.

Differential Revision: https://reviews.llvm.org/D150590