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/tools/dsymutil/DwarfLinker.cpp b/llvm/tools/dsymutil/DwarfLinker.cpp
index 778c760..9c446f4 100644
--- a/llvm/tools/dsymutil/DwarfLinker.cpp
+++ b/llvm/tools/dsymutil/DwarfLinker.cpp
@@ -2666,6 +2666,7 @@
   case dwarf::DW_TAG_packed_type:
   case dwarf::DW_TAG_volatile_type:
   case dwarf::DW_TAG_restrict_type:
+  case dwarf::DW_TAG_atomic_type:
   case dwarf::DW_TAG_interface_type:
   case dwarf::DW_TAG_unspecified_type:
   case dwarf::DW_TAG_shared_type: