[DirectX] Update for API change in llvm#139406 (llvm#139470)

getAsmString returns StringRef now. Fixes build for the DirectX backend.

Author: bogner

llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp

@@ -1974,7 +1974,7 @@ void DXILBitcodeWriter::writeConstants(unsigned FirstVal, unsigned LastVal,
                        unsigned(IA->getDialect() & 1) << 2);
 
       // Add the asm string.
-      const std::string &AsmStr = IA->getAsmString();
+      StringRef AsmStr = IA->getAsmString();
       Record.push_back(AsmStr.size());
       Record.append(AsmStr.begin(), AsmStr.end());