[LLDB] - Add support for DW_FORM_implicit_const.

LLDB does not support this DWARF5 form atm.
At least gcc emits it in some cases when doing optimization
for abbreviations.

As far I can tell, clang does not support it yet, though
the rest LLVM code already knows about it.

The patch adds the support.

Differential revision: https://reviews.llvm.org/D52689

llvm-svn: 344328