driver/toobj.cpp: writeModule: don't use StringRef::data to get a c_s…

…tr (#4554)

The .data() method on a StringRef, returns a const char* to the first
character but there is no guarantee that the string is null
terminated.

In order to get a proper null terminated string it is
necessary to use .str() which will return a C++ std::string which
provided the .c_str() method to get the null terminated C string.

The invalid behavior can be seen by running (where the user doesn't
have acces to the inaccessible folder):
$ ldc2 <<< '' - -od=inaccessible/impossible
Error: failed to create output directory: inaccessible/impossible/__stdin_4416.o

The error message should only include inaccessible/impossible.

Signed-off-by: Andrei Horodniceanu <[email protected]>

driver/toobj.cpp

@@ -378,7 +378,7 @@ void writeModule(llvm::Module *m, const char *filename) {
   if (!directory.empty()) {
     if (auto ec = llvm::sys::fs::create_directories(directory)) {
       error(Loc(), "failed to create output directory: %s\n%s",
-            directory.data(), ec.message().c_str());
+            directory.str().c_str(), ec.message().c_str());
       fatal();
     }
   }