[llvm-diff] add "reset" method to DiffConsumer

[void]

Summary

A DiffConsumer object may be reused, but we'd like to reset it before
the next use.

No functionality change intended.

[kiranchandramohan]

Our buildbot seems to have failed due to an issue here.
https://lab.llvm.org/buildbot/#/builders/160/builds/3567/steps/5/logs/stdio

llvm/tools/llvm-diff/lib/DiffConsumer.cpp

138

context -> contexts?

[void]

In D107985#2942298, @kiranchandramohan wrote:

Our buildbot seems to have failed due to an issue here.
https://lab.llvm.org/buildbot/#/builders/160/builds/3567/steps/5/logs/stdio

Sorry about that. I fixed it in the follow-up patch.

llvm/tools/llvm-diff/lib/DiffConsumer.h

private:
     void printValue(const Value *V, bool isL);
     void header();
     void indent();
 
   public:
     DiffConsumer()
       : out(errs()), Differences(false), Indent(0) {}
 
+    void reset();
     bool hadDifferences() const;
     void enterContext(const Value *L, const Value *R) override;
     void exitContext() override;
     void log(StringRef text) override;
     void logf(const LogBuilder &Log) override;
     void logd(const DiffLogBuilder &Log) override;
   };
 }
 
 #endif

llvm/tools/llvm-diff/lib/DiffConsumer.cpp

void DiffConsumer::header() {
   }
 }
 
 void DiffConsumer::indent() {
   unsigned N = Indent;
   while (N--) out << ' ';
 }
 
+void DiffConsumer::reset() {
+  context.clear();
+  Differences = false;
+  Indent = 0;
+}
+
 bool DiffConsumer::hadDifferences() const {
   return Differences;
 }
 
 void DiffConsumer::enterContext(const Value *L, const Value *R) {
   contexts.push_back(DiffContext(L, R));
   Indent += 2;
 }