DW_TAG_atomic_type was already included in Dwarf.defs and emitted correctly,
however Verifier didn't recognize it as valid.
Thus we introduce the following changes:
* Make DW_TAG_atomic_type valid tag for IR and DWARF (enabled only with -gdwarf-5)
* Add it to related docs
* Add DebugInfo tests
Differential Revision: https://reviews.llvm.org/D26144
llvm-svn: 285624
Victor Leschuk