Add DWARF string debug to clang release notes.

D12353 added inline strings to the DWARF info produced by clang. This
turns out to break some debugging software that assumes that a
DW_TAG_variable *must* come with a DW_AT_name. Add a release note to
broadcast this change.

Reviewed By: paulkirth

Differential Revision: https://reviews.llvm.org/D126224