Add pointer dereference support to probe_read

Allows things like:
```c
int kprobe__foo(struct pt_regs *ctx, u64 *ptr) {
  bpf_trace_printk("%lx\n", *ptr);
```
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

src/cc/frontends/clang/b_frontend_action.cc

@@ -158,6 +158,23 @@ bool ProbeVisitor::VisitBinaryOperator(BinaryOperator *E) {
   }
   return true;
 }
+bool ProbeVisitor::VisitUnaryOperator(UnaryOperator *E) {
+  if (E->getOpcode() != UO_Deref)
+    return true;
+  if (memb_visited_.find(E) != memb_visited_.end())
+    return true;
+  if (!ProbeChecker(E, ptregs_).needs_probe())
+    return true;
+  memb_visited_.insert(E);
+  Expr *sub = E->getSubExpr();
+  string rhs = rewriter_.getRewrittenText(SourceRange(sub->getLocStart(), sub->getLocEnd()));
+  string text;
+  text = "({ typeof(" + E->getType().getAsString() + ") _val; memset(&_val, 0, sizeof(_val));";
+  text += " bpf_probe_read(&_val, sizeof(_val), (u64)";
+  text += rhs + "); _val; })";
+  rewriter_.ReplaceText(SourceRange(E->getLocStart(), E->getLocEnd()), text);
+  return true;
+}
 bool ProbeVisitor::VisitMemberExpr(MemberExpr *E) {
   if (memb_visited_.find(E) != memb_visited_.end()) return true;
 

src/cc/frontends/clang/b_frontend_action.h

@@ -85,6 +85,7 @@ class ProbeVisitor : public clang::RecursiveASTVisitor<ProbeVisitor> {
   bool VisitVarDecl(clang::VarDecl *Decl);
   bool VisitCallExpr(clang::CallExpr *Call);
   bool VisitBinaryOperator(clang::BinaryOperator *E);
+  bool VisitUnaryOperator(clang::UnaryOperator *E);
   bool VisitMemberExpr(clang::MemberExpr *E);
   void set_ptreg(clang::Decl *D) { ptregs_.insert(D); }
  private: