[LLVM] Use DWARFv4 bitfields when tuning for GDB

GDB implemented data_bit_offset in https://sourceware.org/bugzilla/show_bug.cgi?id=12616
which has been present since GDB 8.0.

GCC started using it at GCC 11.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D135583