[LTO] Use StringRef instead of C-style strings in setCodeGenDebugOptions

[chill]

Summary

Fixes an issue with missing nul-terminators and saves us some
string copying, compared to a version which would insert nul-terminators.

[thegameg]

Would it be cleaner to make setCodeGenDebugOptions take an ArrayRef<StringRef>?

[chill]

Yeah, I'd prefer that, but then I'm hesitant to change APIs.

[thegameg]

LGTM, thanks.

llvm/tools/lto/lto.cpp

 bool lto_codegen_compile_to_file(lto_code_gen_t cg, const char **name) {
   maybeParseOptions(cg);
   return !unwrap(cg)->compile_to_file(
       name, DisableVerify, DisableInline, DisableGVNLoadPRE,
       DisableLTOVectorization);
 }
 
 void lto_codegen_debug_options(lto_code_gen_t cg, const char *opt) {
-  std::vector<const char *> Options;
+  // Make a copy of the options, so we can nul-terminate them each individual
+  // one.
+  std::vector<std::string> Storage;
   for (std::pair<StringRef, StringRef> o = getToken(opt); !o.first.empty();
        o = getToken(o.second))
-    Options.push_back(o.first.data());
+    Storage.push_back(o.first.str());
 
+  std::vector<const char *> Options;
+  transform(Storage, back_inserter(Options),
+            [](const std::string &S) { return S.c_str(); });
   unwrap(cg)->setCodeGenDebugOptions(Options);
 }
 
 void lto_codegen_debug_options_array(lto_code_gen_t cg,
                                      const char *const *options, int number) {
   unwrap(cg)->setCodeGenDebugOptions(makeArrayRef(options, number));
 }