[ASTWriter] Use zlib::compress which operates on std::unique_ptr<uint8_t[]>

[MaskRay]

This depends on D50223

[MaskRay]

https://reviews.llvm.org/D50223 has been changed to improve zlib::compress's current SmallVectorImpl interface. This is no longer relevant.

lib/Serialization/ASTWriter.cpp

 
 static void emitBlob(llvm::BitstreamWriter &Stream, StringRef Blob,
                      unsigned SLocBufferBlobCompressedAbbrv,
                      unsigned SLocBufferBlobAbbrv) {
   using RecordDataType = ASTWriter::RecordData::value_type;
 
   // Compress the buffer if possible. We expect that almost all PCM
   // consumers will not want its contents.
-  SmallString<0> CompressedBuffer;
+  std::unique_ptr<uint8_t[]> CompressedData;
+  size_t CompressedSize;
   if (llvm::zlib::isAvailable()) {
-    llvm::Error E = llvm::zlib::compress(Blob.drop_back(1), CompressedBuffer);
+    llvm::Error E =
+        llvm::zlib::compress(Blob.drop_back(1), CompressedData, CompressedSize);
     if (!E) {
       RecordDataType Record[] = {SM_SLOC_BUFFER_BLOB_COMPRESSED,
                                  Blob.size() - 1};
-      Stream.EmitRecordWithBlob(SLocBufferBlobCompressedAbbrv, Record,
-                                CompressedBuffer);
+      Stream.EmitRecordWithBlob(
+          SLocBufferBlobCompressedAbbrv, Record,
+          StringRef((char *)CompressedData.get(), CompressedSize));
       return;
     }
     llvm::consumeError(std::move(E));
   }
 
   RecordDataType Record[] = {SM_SLOC_BUFFER_BLOB};
   Stream.EmitRecordWithBlob(SLocBufferBlobAbbrv, Record, Blob);
 }