[SourceKit] Use WriteableMemoryBuffer to do the const_cast dirty work.

Based on PR feedback from @CodaFi

Author: rayfix

tools/SourceKit/tools/sourcekitd/lib/API/sourcekitdAPI-XPC.cpp

@@ -244,10 +244,10 @@ void ResponseBuilder::Dictionary::setCustomBuffer(
       SourceKit::UIdent Key,
       CustomBufferKind Kind, std::unique_ptr<llvm::MemoryBuffer> MemBuf) {
 
-  std::unique_ptr<llvm::MemoryBuffer> CustomBuf;
+  std::unique_ptr<llvm::WritableMemoryBuffer> CustomBuf;
   CustomBuf = llvm::WritableMemoryBuffer::getNewUninitMemBuffer(
       sizeof(uint64_t) + MemBuf->getBufferSize());
-  char *BufPtr = const_cast<char*>(CustomBuf->getBufferStart());
+  char *BufPtr = CustomBuf->getBufferStart();
   *reinterpret_cast<uint64_t*>(BufPtr) = (uint64_t)Kind;
   BufPtr += sizeof(uint64_t);
   memcpy(BufPtr, MemBuf->getBufferStart(), MemBuf->getBufferSize());