fix compilation error with latest llvm12 trunk
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 <e76892bbe783db2f627de1232e139db4de43da60@fb.com>

brendangregg/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc