fix compilation error with latest llvm12 trunk

With latest llvm trunk (llvm12), I hit the following compilation
error:
  ...
  [ 17%] Building CXX object src/cc/frontends/b/CMakeFiles/b_frontend.dir/codegen_llvm.cc.o
  /home/yhs/work/bcc/src/cc/frontends/b/codegen_llvm.cc: In member function
     ‘virtual ebpf::StatusTuple ebpf::cc::CodegenLLVM::visit_table_decl_stmt_node(ebpf::cc::TableDeclStmtNode*)’:
  /home/yhs/work/bcc/src/cc/frontends/b/codegen_llvm.cc:1122:37:
     error: ‘class llvm::Module’ has no member named ‘getTypeB yName’; did you mean ‘getName’?
     StructType *decl_struct = mod_->getTypeByName("_struct." + n->id_->name_);
                                     ^~~~~~~~~~~~~
                                     getName

This is due to llvm patch https://reviews.llvm.org/D78793 which
changed how to use getTypeByName(). This patch adjusted the usage
in bcc based on this patch.

Signed-off-by: Yonghong Song <[email protected]>

src/cc/frontends/b/codegen_llvm.cc

@@ -1119,7 +1119,11 @@ StatusTuple CodegenLLVM::visit_table_decl_stmt_node(TableDeclStmtNode *n) {
  StructType *key_stype, *leaf_stype;
  TRY2(lookup_struct_type(n->key_type_, &key_stype));
  TRY2(lookup_struct_type(n->leaf_type_, &leaf_stype));
+#if LLVM_MAJOR_VERSION >= 12
+ StructType *decl_struct = StructType::getTypeByName(mod_->getContext(), "_struct." + n->id_->name_);
+#else
  StructType *decl_struct = mod_->getTypeByName("_struct." + n->id_->name_);
+#endif
  if (!decl_struct)
  decl_struct = StructType::create(ctx(), "_struct." + n->id_->name_);
  if (decl_struct->isOpaque())
@@ -1182,7 +1186,11 @@ StatusTuple CodegenLLVM::visit_func_decl_stmt_node(FuncDeclStmtNode *n) {
  StructType *stype;
  //TRY2(lookup_struct_type(formal, &stype));
  auto var = (StructVariableDeclStmtNode *)formal;
+#if LLVM_MAJOR_VERSION >= 12
+ stype = StructType::getTypeByName(mod_->getContext(), "_struct." + var->struct_id_->name_);
+#else
  stype = mod_->getTypeByName("_struct." + var->struct_id_->name_);
+#endif
  if (!stype) return mkstatus_(n, "could not find type %s", var->struct_id_->c_str());
  formals.push_back(PointerType::getUnqual(stype));
  } else {