DebugInfo: make DW_TAG_atomic_type valid

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
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 7eea517..a8a1df3 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -864,6 +864,7 @@
                N.getTag() == dwarf::DW_TAG_const_type ||
                N.getTag() == dwarf::DW_TAG_volatile_type ||
                N.getTag() == dwarf::DW_TAG_restrict_type ||
+               N.getTag() == dwarf::DW_TAG_atomic_type ||
                N.getTag() == dwarf::DW_TAG_member ||
                N.getTag() == dwarf::DW_TAG_inheritance ||
                N.getTag() == dwarf::DW_TAG_friend,