Frontend: Simplify handling of non-seeking streams in CompilerInstance, NFC

Add a new `raw_pwrite_ostream` variant, `buffer_unique_ostream`, which
is like `buffer_ostream` but with unique ownership of the stream it's
wrapping. Use this in CompilerInstance to simplify the ownership of
non-seeking output streams, avoiding logic sprawled around to deal with
them specially.

This also simplifies future work to encapsulate output files in a
different class.

Differential Revision: https://reviews.llvm.org/D93260

include/llvm/Support/raw_ostream.h

@@ -687,6 +687,18 @@ public:
   ~buffer_ostream() override { OS << str(); }
 };
 
+class buffer_unique_ostream : public raw_svector_ostream {
+  std::unique_ptr<raw_ostream> OS;
+  SmallVector<char, 0> Buffer;
+
+  virtual void anchor() override;
+
+public:
+  buffer_unique_ostream(std::unique_ptr<raw_ostream> OS)
+      : raw_svector_ostream(Buffer), OS(std::move(OS)) {}
+  ~buffer_unique_ostream() override { *OS << str(); }
+};
+
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_RAW_OSTREAM_H

lib/Support/raw_ostream.cpp

@@ -987,3 +987,5 @@ void raw_null_ostream::pwrite_impl(const char *Ptr, size_t Size,
 void raw_pwrite_stream::anchor() {}
 
 void buffer_ostream::anchor() {}
+
+void buffer_unique_ostream::anchor() {}