Make circular_raw_ostream delegate is_displayed to contained stream

[troyj]

Summary

raw_ostream has an is_displayed() member function that determines if the stream is connected to a console for display or is connected to a file/pipe. By default, is_displayed() return false, and derived classes like raw_fd_ostream override it. Because circular_raw_ostream wraps another stream, its result for is_displayed() should be the same as that stream.

[jkorous]

LGTM

llvm/include/llvm/Support/circular_raw_ostream.h

public:
 
     ~circular_raw_ostream() override {
       flush();
       flushBufferWithBanner();
       releaseStream();
       delete[] BufferArray;
     }
 
+    bool is_displayed() const override {
+      return TheStream->is_displayed();
+    }
+
     /// setStream - Tell the circular_raw_ostream to output a
     /// different stream.  "Owns" tells circular_raw_ostream whether
     /// it should take responsibility for managing the underlying
     /// stream.
     ///
     void setStream(raw_ostream &Stream, bool Owns = REFERENCE_ONLY) {
       releaseStream();
       TheStream = &Stream;