[DFSan] Add flag to insert event callbacks.

[morehouse]

Summary

For now just insert the callback for stores, similar to how MSan tracks
origins. In the future we may want to add callbacks for loads, memcpy,
function calls, CMPs, etc.

[kcc]

Code LGTM, but please add some comments near the flag definition.

[eugenis]

Do you want to also pass the store address to the callback?

[morehouse]

In D75312#1897073, @eugenis wrote:

Do you want to also pass the store address to the callback?

Maybe? For the immediate use case (i.e. getting a stack trace) it's probably not needed, but maybe the address would be useful for something else?

I noticed MSan origins reports don't give the store address, so I figured it wasn't too important.

[morehouse]

• Add comment explaining the new flag.

[eugenis]

If MSan remembered store addresses, it would very fast run out of origin ids.

Since this is an experimental API without a frontend flag, I'd say do what is useful right now, and add more info later if it becomes necessary.

LGTM

[morehouse]

• Fix diffbase.

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

static cl::opt<bool> ClCombinePointerLabelsOnStore(
     cl::Hidden, cl::init(false));
 
 static cl::opt<bool> ClDebugNonzeroLabels(
     "dfsan-debug-nonzero-labels",
     cl::desc("Insert calls to __dfsan_nonzero_label on observing a parameter, "
              "load or return with a nonzero label"),
     cl::Hidden);
 
+// Experimental feature that inserts callbacks for certain data events.
+// Currently callbacks are only inserted for stores.
+//
+// If this flag is set to true, the user must provide definitions for the
+// following callback functions:
+//   void __dfsan_store_callback(dfsan_label Label);
 static cl::opt<bool> ClEventCallbacks(
     "dfsan-event-callbacks",
     cl::desc("Insert calls to __dfsan_*_callback functions on data events."),
     cl::Hidden, cl::init(false));
 
 static StringRef GetGlobalTypeString(const GlobalValue &G) {
   // Types of GlobalVariables are always pointer types.
   Type *GType = G.getValueType();