Migrate to newer API.

Following function (left) is deprecated, use the right version
instead:

llvm::sys::Path::getLast     -> llvm::sys::path::filename
llvm::sys::Path::getBasename -> llvm::sys::path::stem
llvm::sys::Path::getDirname  -> llvm::sys::path::parent_path
diff --git a/slang_rs.cpp b/slang_rs.cpp
index c5aadff..7945f95 100644
--- a/slang_rs.cpp
+++ b/slang_rs.cpp
@@ -232,9 +232,8 @@
                                        const clang::SourceManager &SourceMgr) {
   clang::FullSourceLoc FSL(FD->getLocStart(), SourceMgr);
   clang::PresumedLoc PLoc = SourceMgr.getPresumedLoc(FSL);
-  llvm::sys::Path HeaderFilename(PLoc.getFilename());
 
-  return IsRSHeaderFile(HeaderFilename.getLast().data());
+  return IsRSHeaderFile(llvm::sys::path::filename(PLoc.getFilename()).data());
 }
 
 SlangRS::SlangRS() : Slang(), mRSContext(NULL), mAllowRSPrefix(false) {