[RFC][BPF] Handle DW_TAG_atomic_type properly

Make change in BTFDebug.cpp to handle DW_TAG_atomic_type properly.
Otherwise, a type like
   _Atomic int i; // global
the dwarf type chain atomic->int
Since DW_TAG_atomic_type is not processed BTF generation will stop
at atomic modifier and BTF will encode 'i' as void type.

Similar for type like
  volatile _Atomic int *p;
the dwarf type chain ptr->volatile->atomic->int
Since atomic type is not processed and BTF generation will stop at
atomic type, the eventual BTF type will be
  ptr->volatile->void
which is incorrect.

This patch fixed the above two patterns by skipping
DW_TAG_atomic_type. There could be more cases which I will try
to fix those with more test cases. This is a RFC patch and the
current implementation is good enough to run kernel selftests
with _Atomic usage ([1]).

In kernel selftest arena_atomics.c, the new bpf code looks like

```
_Atomic __u64 __arena_global and64_value = (0x110ull << 32);
_Atomic __u32 __arena_global and32_value = 0x110;

SEC("raw_tp/sys_enter")
int and(const void *ctx)
{
	...
        __c11_atomic_fetch_and(&and64_value, 0x011ull << 32, memory_order_relaxed);
        __c11_atomic_fetch_and(&and32_value, 0x011, memory_order_relaxed);
	...

        return 0;
}
```

The skel file arena_atomics.skel.h has
```
struct arena_atomics__arena {
	...
	__u64 and64_value;
	__u32 and32_value;
	...
} *arena;
```

  [1] https://lore.kernel.org/bpf/[email protected]/

llvm/lib/Target/BPF/BTFDebug.cpp

@@ -91,6 +91,12 @@ void BTFTypeDerived::completeType(BTFDebug &BDebug) {
 
   // The base type for PTR/CONST/VOLATILE could be void.
   const DIType *ResolvedType = DTy->getBaseType();
+  if (ResolvedType) {
+    const auto *DerivedTy = dyn_cast<DIDerivedType>(ResolvedType);
+    if (DerivedTy && DerivedTy->getTag() == dwarf::DW_TAG_atomic_type)
+      ResolvedType = DerivedTy->getBaseType();
+  }
+
   if (!ResolvedType) {
     assert((Kind == BTF::BTF_KIND_PTR || Kind == BTF::BTF_KIND_CONST ||
             Kind == BTF::BTF_KIND_VOLATILE) &&
@@ -800,6 +806,10 @@ void BTFDebug::visitDerivedType(const DIDerivedType *DTy, uint32_t &TypeId,
                                 bool CheckPointer, bool SeenPointer) {
   unsigned Tag = DTy->getTag();
 
+  if (Tag == dwarf::DW_TAG_atomic_type)
+    return visitTypeEntry(DTy->getBaseType(), TypeId, CheckPointer,
+                          SeenPointer);
+
   /// Try to avoid chasing pointees, esp. structure pointees which may
   /// unnecessary bring in a lot of types.
   if (CheckPointer && !SeenPointer) {
@@ -1444,8 +1454,15 @@ void BTFDebug::processGlobals(bool ProcessingMapDef) {
       DIGlobal = GVE->getVariable();
       if (SecName.starts_with(".maps"))
         visitMapDefType(DIGlobal->getType(), GVTypeId);
-      else
-        visitTypeEntry(DIGlobal->getType(), GVTypeId, false, false);
+      else {
+        const DIType *Ty = DIGlobal->getType();
+        if (Ty) {
+          auto *DTy = dyn_cast<DIDerivedType>(Ty);
+          if (DTy && DTy->getTag() == dwarf::DW_TAG_atomic_type)
+            Ty = DTy->getBaseType();
+        }
+        visitTypeEntry(Ty, GVTypeId, false, false);
+      }
       break;
     }