Use None instead of an explicit constructor.

And remove a needless 'const' since ArrayRef is immutable.
Both as suggested by dblaikie on cfe-commits.

llvm-svn: 247995
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 8ec636a..f231282 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -1171,7 +1171,7 @@
 /// triple.
 void Generic_GCC::GCCInstallationDetector::init(
     const Driver &D, const llvm::Triple &TargetTriple, const ArgList &Args,
-    const ArrayRef<std::string> ExtraTripleAliases) {
+    ArrayRef<std::string> ExtraTripleAliases) {
   llvm::Triple BiarchVariantTriple = TargetTriple.isArch32Bit()
                                          ? TargetTriple.get64BitArchVariant()
                                          : TargetTriple.get32BitArchVariant();