[mlir] Relax requirement on memory buffer creation.

[jpienaar]

Summary

parseSourceString does not require null-terminated string, hence
requirement on memory buffer can be relaxed.

mlir/lib/Parser/Parser.cpp

if (failed(loadSourceFileBuffer(filename, *sourceMgr, config.getContext())))
     return failure();
   return parseSourceFile(sourceMgr, block, config, sourceFileLoc);
 }
 
 LogicalResult mlir::parseSourceString(llvm::StringRef sourceStr, Block *block,
                                       const ParserConfig &config,
                                       StringRef sourceName,
                                       LocationAttr *sourceFileLoc) {
-  auto memBuffer = llvm::MemoryBuffer::getMemBuffer(sourceStr, sourceName);
+  auto memBuffer =
+      llvm::MemoryBuffer::getMemBuffer(sourceStr, sourceName,
+                                       /*RequiresNullTerminator=*/false);
   if (!memBuffer)
     return failure();
 
   llvm::SourceMgr sourceMgr;
   sourceMgr.AddNewSourceBuffer(std::move(memBuffer), SMLoc());
   return parseSourceFile(sourceMgr, block, config, sourceFileLoc);
 }