[MIPS] initFeatureMap() to handle empty string argument

SUMMARY:
    This patch sets CPU string to its default value when it is not supplied by caller.
    
    Reviewers: vkalintiris, dsanders
    Subscribers: mohit.bhakkad, sagar, jaydeep, cfe-commits
    Differential Revision: http://reviews.llvm.org/D16139

llvm-svn: 262691

clang/lib/Basic/Targets.cpp

@@ -6689,6 +6689,8 @@ public:
   initFeatureMap(llvm::StringMap<bool> &Features, DiagnosticsEngine &Diags,
                  StringRef CPU,
                  const std::vector<std::string> &FeaturesVec) const override {
+    if (CPU.empty())
+      CPU = getCPU();
     if (CPU == "octeon")
       Features["mips64r2"] = Features["cnmips"] = true;
     else