[DirectX] Update DXILBitcodeWriter for llvm#162255 (llvm#162531)

Fix the build with the DirectX backend enabled by using `getUnversionedName()`
to match the updates of other similar uses of `getSourceLanguage()`.

Author: bogner

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

@@ -1507,7 +1507,7 @@ void DXILBitcodeWriter::writeDICompileUnit(const DICompileUnit *N,
                                            SmallVectorImpl<uint64_t> &Record,
                                            unsigned Abbrev) {
   Record.push_back(N->isDistinct());
-  Record.push_back(N->getSourceLanguage());
+  Record.push_back(N->getSourceLanguage().getUnversionedName());
   Record.push_back(VE.getMetadataOrNullID(N->getFile()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawProducer()));
   Record.push_back(N->isOptimized());