Fix issues in BTFDebug for potential missing __bpf_trap btf

The funciton __bpf_trap is created during selectiondag lowering.
But the function usage could be removed during later machine-level
optimization. In such cases, we will generate btf for __bpf_trap
in endModule().

Author: Yonghong Song

llvm/lib/Target/BPF/BTFDebug.cpp

@@ -1622,6 +1622,13 @@ void BTFDebug::endModule() {
   // Collect global types/variables except MapDef globals.
   processGlobals(false);
 
+  // In case that BPF_TRAP usage is removed during machine-level optimization,
+  // generate btf for BPF_TRAP function here.
+  for (const Function &F : *MMI->getModule()) {
+    if (F.getName() == BPF_TRAP)
+      processFuncPrototypes(&F);
+  }
+
   for (auto &DataSec : DataSecEntries)
     addType(std::move(DataSec.second));