win: Omit ".exe" from clang and clang-cl driver-level diagnostics.

[thakis]

Summary

Like https://reviews.llvm.org/D51133 but for clang.

[hans]

Nice, thanks!

[thakis]

r340498, thanks!

clang/tools/driver/driver.cpp

static void SetBackdoorDriverOutputsFromEnvVars(Driver &TheDriver) {
   if (TheDriver.CCLogDiagnostics)
     TheDriver.CCLogDiagnosticsFilename = ::getenv("CC_LOG_DIAGNOSTICS_FILE");
 }
 
 static void FixupDiagPrefixExeName(TextDiagnosticPrinter *DiagClient,
                                    const std::string &Path) {
   // If the clang binary happens to be named cl.exe for compatibility reasons,
   // use clang-cl.exe as the prefix to avoid confusion between clang and MSVC.
-  StringRef ExeBasename(llvm::sys::path::filename(Path));
-  if (ExeBasename.equals_lower("cl.exe"))
-    ExeBasename = "clang-cl.exe";
+  StringRef ExeBasename(llvm::sys::path::stem(Path));
+  if (ExeBasename.equals_lower("cl"))
+    ExeBasename = "clang-cl";
   DiagClient->setPrefix(ExeBasename);
 }
 
 // This lets us create the DiagnosticsEngine with a properly-filled-out
 // DiagnosticOptions instance.
 static DiagnosticOptions *
 CreateAndPopulateDiagOpts(ArrayRef<const char *> argv) {
   auto *DiagOpts = new DiagnosticOptions;