[lldb] Change return type of FileSpec::GetFileNameExtension

These don't really need to be in ConstStrings. It's nice that comparing
ConstStrings is fast (just a pointer comparison) but the cost of
creating the ConstString usually already includes the cost of doing a
StringRef comparison anyway, so this is just extra work and extra memory
consumption for basically no benefit.

Differential Revision: https://reviews.llvm.org/D149300

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -306,7 +306,7 @@ void ScriptInterpreterPython::SharedLibraryDirectoryHelper(
   // On windows, we need to manually back out of the python tree, and go into
   // the bin directory. This is pretty much the inverse of what ComputePythonDir
   // does.
-  if (this_file.GetFileNameExtension() == ConstString(".pyd")) {
+  if (this_file.GetFileNameExtension() == ".pyd") {
     this_file.RemoveLastPathComponent(); // _lldb.pyd or _lldb_d.pyd
     this_file.RemoveLastPathComponent(); // lldb
     llvm::StringRef libdir = LLDB_PYTHON_RELATIVE_LIBDIR;