swift-scan-test: adjust frontend detection for Windows

Ensure that we account for file system arc separators and extensions by
using the LLVM functions rather than trying to simply do string
matching. This allows us to properly handle inputs on Windows.

Author: compnerd

tools/swift-scan-test/swift-scan-test.cpp

@@ -23,6 +23,7 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/ThreadPool.h"
 
@@ -264,8 +265,12 @@ static int action_scan_dependency(std::vector<const char *> &Args,
 
 static std::vector<const char *>
 createArgs(ArrayRef<std::string> Cmd, StringSaver &Saver, Actions Action) {
-  if (!Cmd.empty() && StringRef(Cmd.front()).ends_with("swift-frontend"))
-    Cmd = Cmd.drop_front();
+  if (!Cmd.empty()) {
+    llvm::SmallString<261> path;
+    llvm::sys::path::native(Twine{Cmd.front()}, path);
+    if (llvm::sys::path::filename(path).starts_with("swift-frontend"))
+      Cmd = Cmd.drop_front();
+  }
 
   // Quote all the arguments before passing to scanner. The scanner is currently
   // tokenize the command-line again before parsing.