Interpret an empty code model as no code model, or the default code model

Author: rnk

clang/lib/CodeGen/BackendUtil.cpp

@@ -304,7 +304,7 @@ getCodeModel(const CodeGenOptions &CodeGenOpts) {
                            .Case("kernel", llvm::CodeModel::Kernel)
                            .Case("medium", llvm::CodeModel::Medium)
                            .Case("large", llvm::CodeModel::Large)
-                           .Case("default", ~1u)
+                           .Cases("default", "", ~1u)
                            .Default(~0u);
   assert(CodeModel != ~0u && "invalid code model!");
   if (CodeModel == ~1u)