[Coverage] Fix use-after free in coverage emission

[efriedma]

Summary

Fixes regression from r320533.

This fixes the undefined behavior, but I'm not sure it's really right... I think we end up with missing coverage for code in modules.

[vsk]

Thanks, this lgtm as a stopgap.

As I mentioned offline, I think the goal should be to have non-modules and modules-enabled builds of a project produce identical coverage reports. I'll look into what exactly we're adding to the deferred decls map while iterating over it, and see if we're dropping any coverage mappings.

cfe/trunk/lib/CodeGen/CodeGenModule.cpp

void CodeGenModule::ClearUnusedCoverageMapping(const Decl *D) {
   auto I = DeferredEmptyCoverageMappingDecls.find(D);
   if (I == DeferredEmptyCoverageMappingDecls.end())
     DeferredEmptyCoverageMappingDecls[D] = false;
   else
     I->second = false;
 }
 
 void CodeGenModule::EmitDeferredUnusedCoverageMappings() {
-  for (const auto &Entry : DeferredEmptyCoverageMappingDecls) {
+  // We call takeVector() here to avoid use-after-free.
+  // FIXME: DeferredEmptyCoverageMappingDecls is getting mutated because
+  // we deserialize function bodies to emit coverage info for them, and that
+  // deserializes more declarations. How should we handle that case?
+  for (const auto &Entry : DeferredEmptyCoverageMappingDecls.takeVector()) {
     if (!Entry.second)
       continue;
     const Decl *D = Entry.first;
     switch (D->getKind()) {
     case Decl::CXXConversion:
     case Decl::CXXMethod:
     case Decl::Function:
     case Decl::ObjCMethod: {