fix compilation issues with latest llvm12 trunk

With latest llvm12 trunk, we got two compilation bugs.

Bug iovisor#1:
  /home/yhs/work/bcc/src/cc/frontends/clang/b_frontend_action.cc:
     In member function ‘void ebpf::BFrontendAction::DoMiscWorkAround()’:
  /home/yhs/work/bcc/src/cc/frontends/clang/b_frontend_action.cc:1706:31:
     error: ‘class clang::SourceManage’ has no member named ‘getBuffer’; did you mean ‘getBufferData’?
     rewriter_->getSourceMgr().getBuffer(rewriter_->getSourceMgr().getMainFileID())->getBufferSize(),
                               ^~~~~~~~~
                               getBufferData

  This is due to upstream change https://reviews.llvm.org/D89394.
  To fix, follow upstream examples in https://reviews.llvm.org/D89394.

Bug iovisor#2:
  /home/yhs/work/bcc/src/cc/bpf_module.cc: In member function ‘int ebpf::BPFModule::finalize()’:
  /home/yhs/work/bcc/src/cc/bpf_module.cc:470:11:
    error: ‘class llvm::EngineBuilder’ has no member named ‘setUseOrcMCJITReplacement’
   builder.setUseOrcMCJITReplacement(false);
           ^~~~~~~~~~~~~~~~~~~~~~~~~

  This is due to upstream
    llvm/llvm-project@6154c41

  It seems builder.setUseOrcMCJITReplacement() is not needed any more. So just remove it
  from bcc.

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

src/cc/bpf_module.cc

@@ -466,7 +466,9 @@ int BPFModule::finalize() {
  builder.setErrorStr(&err);
  builder.setMCJITMemoryManager(ebpf::make_unique<MyMemoryManager>(sections_p));
  builder.setMArch("bpf");
+#if LLVM_MAJOR_VERSION <= 11
  builder.setUseOrcMCJITReplacement(false);
+#endif
  engine_ = unique_ptr<ExecutionEngine>(builder.create());
  if (!engine_) {
  fprintf(stderr, "Could not create ExecutionEngine: %s\n", err.c_str());

src/cc/bpf_module_rw_engine.cc

@@ -356,7 +356,9 @@ unique_ptr<ExecutionEngine> BPFModule::finalize_rw(unique_ptr<Module> m) {
  string err;
  EngineBuilder builder(move(m));
  builder.setErrorStr(&err);
+#if LLVM_MAJOR_VERSION <= 11
  builder.setUseOrcMCJITReplacement(false);
+#endif
  auto engine = unique_ptr<ExecutionEngine>(builder.create());
  if (!engine)
  fprintf(stderr, "Could not create ExecutionEngine: %s\n", err.c_str());

src/cc/frontends/clang/b_frontend_action.cc

@@ -1700,7 +1700,11 @@ void BFrontendAction::DoMiscWorkAround() {
  false);
 
  rewriter_->getEditBuffer(rewriter_->getSourceMgr().getMainFileID()).InsertTextAfter(
+#if LLVM_MAJOR_VERSION >= 12
+ rewriter_->getSourceMgr().getBufferOrFake(rewriter_->getSourceMgr().getMainFileID()).getBufferSize(),
+#else
  rewriter_->getSourceMgr().getBuffer(rewriter_->getSourceMgr().getMainFileID())->getBufferSize(),
+#endif
  "\n#include <bcc/footer.h>\n");
 }