Make StringSet::insert return pair<iterator, bool> like other self-associative containers

StringSet is still a bit dodgy in that it exposes the raw iterator of
the StringMap parent, which exposes the weird detail that StringSet
actually has a 'value'... but anyway, this is useful for a handful of
clients that want to reference the newly inserted/persistent string data
in the StringSet/Map/Entry/thing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222302 91177308-0d34-0410-b5e6-96231b3b80d8

tools/opt/BreakpointPrinter.cpp

@@ -62,7 +62,7 @@ struct BreakpointPrinter : public ModulePass {
           continue;
         getContextName(SP.getContext().resolve(TypeIdentifierMap), Name);
         Name = Name + SP.getDisplayName().str();
-        if (!Name.empty() && Processed.insert(Name)) {
+        if (!Name.empty() && Processed.insert(Name).second) {
           Out << Name << "\n";
         }
       }